Go to PokerStars installation directory (Usually it is located at C:/Program Files (x86)/PokerStars)
Go to backup folder
Select all the files by pressing CTRL + A
Copy all the files you’ve selected by pressing CTRL + C
Go back to PokerStars folder
Paste all the files by pressing CTRL + V, select "Replace files in destination folder"
Run PokerStars using PokerStars.exe file from PokerStars folder (Not via the standard shortcut on the desktop, otherwise the client will be updated again)