_%ProgramFiles(x86)%\Riot Games\Riot Client_
Click on the "X" if you get a prompt to install the new client again.
EDIT: I forgot to mention that you also have to launch the game via "LeagueClient.exe". When you install the new client it creates a shortcut that's linked to it, so upon deleting it that shortcut won't work anymore. Just go to your PBE folder and create a new shortcut for "LeagueClient.exe".
Post your PBE bug reports here: something not functioning as intended? Store is broken? A champion's ability is not working? etc
Need help? Read our [bug reporting guide](http://community.pbe.leagueoflegends.com/c/bugs/CF5EEDC8FF5AC1A896F517E2C7AE4650C472807F-how-to-report-bugs-on-the-pbe).
Comments about things you don't like or suggestions for improvements go in Feature Feedback.