Learn more about the Legacy client's retirement here!
The client affects every aspect of your League experience: the store, lobbies, chat, champ select, everything. Most games don't ever need to replace their client. But League has been happily chugging along for the better part of the decade, and some of its parts have started to show their age.
We've pushed our loyal little client to its limits, but it's built on old, outdated tech--it just isn't designed to support the things we want to do for League's future. So, we're rebuilding it! The client update is a from-the-ground-up rework of League's home.
The client update is a way for us to modernize the client's visuals and fix long-standing bugs, but one of the biggest reasons we're replacing the legacy client is so Rioters can get faster at shipping features players want. The legacy client's old, outdated tech holds developers back from making new features in a timely manner. The reasons are complicated, but the short and simple explanation is that the updated client runs on newer, more stable technology that lets Rioters make code changes without tripping over each other.
Throughout the client update alpha and beta we've had to develop features for both the old, slow-as-hell (development-wise) legacy client and the updated client. That's twice the work for every feature, which just isn't sustainable in the long term. By fully replacing the legacy client, however, Rioters will finally be unlocked to work on the new features players have been demanding.
The Basics
How do I download and install the updated client?
Once the transfer process begins, the updated client will appear as a simple, automated download whenever you next launch League of Legends.
I'm already using the updated client, do I need to do anything to update?
Nope! You can just keep playing. It will automatically update and fully replace any legacy client files on your computer.
Can I keep playing on the legacy client instead of upgrading?
No, every League player has to download the update before launching the client
Should I uninstall the legacy client?
Do not uninstall the legacy client! Uninstalling the legacy client will uninstall your entire LoL folder, so you'd just have to reinstall all over again. After beta is over, once the transition has gone smoothly for everyone, we'll remove legacy client files automatically across several patches.
If you're trying to clear up space on your hard drive, feel free to delete the lol_air_client folder located in your install directory. The default location is:
Windows: C:\Riot Games\League of Legends\RADS\projects\lol_air_client
Mac: /Applications/League of Legends.app/Contents/LoL/RADS/projects/lol_air_client
How will the updated client rollout happen?
When the rollout begins in your region, it will happen in four phases:
Phase 1: A small group of randomly-selected players will be automatically upgraded to the updated client, although they can still launch the legacy client from the login screen.
Phase 2: A short time after phase 1, all players will be automatically switched to the updated client, without access to the legacy client.
Phase 3: Everyone else (all players, all regions) will be automatically switched to the updated client, and the legacy client will be officially retired. This will happen automatically when players launch League without requiring the download of additional files (beyond that of the regular patch for that week). However, we'll temporarily keep the legacy client files on your computer in case of an emergency. Beginning with this patch, we'll replace the legacy client with the updated client on all publicly-hosted game download pages.
Phase 4: We'll automatically remove legacy client files from your machine to free up space.
This gradual rollout will give us time to monitor player feedback and make adjustments in response. We'll extend or delay the rollout if we find that more significant adjustments are necessary.
Features
Does the updated client have the same features as the legacy client?
Yep, plus and minus a few exceptions. All new client features like Replays and Practice Tool are going to continue to be exclusive to the updated client. One legacy client feature that isn’t going to be immediately available is the item set creator tool. Early on in the client alpha we announced that we weren't planning to rebuild the legacy client's built-in item set creator tool (although the updated client still supports custom item sets). Players passionately asked us to reconsider, so we did.
We now intend to rebuild the item set creator tool, but haven't yet started development work on it, so it could take quite some time even after we replace the legacy client. But it is finally back on our to-do list. Of course, replacing the legacy client still comes first. However, your existing item sets will still continue to work and our third-party API will still allow you to download custom item sets from community developers for in-game use.
Does the updated client use Adobe Air?
Nope! The updated client is built from the ground up, which lets us more effectively develop, ship, and support new features (like Replays and the Practice Tool). You can read more about the tech behind the client on our engineering blog.
Can I change my in-game settings in the client?
Absolutely. This is a new feature exclusive to the updated client. Use the gear in the top right of the client: 
There have been a few tricks over the years to reskin the legacy client or use a different language pack. Are modifications like this supported in the updated client?
Client modifications weren't supported on the legacy client, and won't be supported in the updated client.
Tech
Are there any known issues with the updated client?
We'll keep tabs on some known issues here.
How do I report bugs?
You can use the bug icon that's always at the bottom of the friends list.
How do I give feedback on the client update?
We'll periodically send out surveys to random users. In addition, you can share and discuss feedback on the client discusison boards here: EUW | EUNE | NA | OCE
How big is the download for the updated client?
Around 1.3 gigabytes. In the past few patches, we've been preloading the files for the updated client in smaller batches. So even if you haven't updated your client yet, you've already downloaded most of the data required.
What will happen to the legacy client's files on my computer?
We'll keep the legacy client's files around for a bit after the end of beta until everyone's transition into the updated client has gone smoothly. Within a few patches, we'll remove them automatically.
Does the updated client have different system requirements?
The updated client has the same minimum and recommended system requirements as the legacy client. You can check these requirements here.
Because the updated client features some new animations, we've added an optional Low Spec Mode for users having performance issues with the client. Enabling Low Spec Mode disables animations and other resource-hogging visual elements without restricting access to any of the client's actual features.
Account
Will my account's level or any of my stats be reset when I switch to the updated client?
No. The only thing changing about the updated client is the client itself. The account you use and everything you've unlocked will stay the same.
I'm not able to log into my account!
The updated client uses the same username and password as the legacy client. Double check that you're logging in with the same username you use to log into League of Legends normally. If you still can't log in, please visit the support site for more help.
Legacy Client Retirement
There are some things I don't like about the new client. Why replace it now?
We've talked about a bunch of the reasons here.
Short version: There are huge development costs to maintaining two clients. Killing legacy unblocks Riot to go full steam ahead on new client feature development. Until we move on from the legacy client, things like 10 bans just can't happen (or we'd have to develop them twice).
Can I go back to using the legacy client after it's retired?
No, the goal of the client rollout is to replace the legacy client.
I gave the updated client a try and it was too laggy/slow
Performance and stability have come a long way since the beginning of the year. We've introduced a Low-Spec mode that automatically detects your specs and defaults to recommended settings to ensure a smooth experience.
If the updated client is slow on your computer, make sure you have Low-Spec mode enabled. If you believe the client may be causing lower FPS on your machine during a game, you can also toggle the option to close the client during game to improve performance while in a match. Note that it will take longer to get to the end of game screen after a match finishes.
The updated client feels laggy even after enabling Low-Spec mode
We’re going to continue making improvements to performance, so if your client continues to lag after optimizing your settings, please send us your logs! Low performance logs will help us pinpoint why you’re lagging so we know what to improve.
Download and run the Hextech Repair Tool and turn on "Gather System & LoL Logs". This will compact all the necessary logs into a zipped file. Submit your logs by creating a ticket, and let us know the details about where your performance is suffering the most.