Make sure to save SEED!
If you already set up password, unlock before backup
Advanced --> Accounts --> Backup/clipboard wallet Seed
Then paste with Ctrl+V in Notepad and save in safe place
64 digits and letters
Check twice! Seed is the most important thing to restore your accounts

Close wallet and make sure it's closed in Task Manager Processes (rai_wallet.exe)
Download and extract files from archive to %LOCALAPPDATA%\RaiBlocks with replacement
Win64 (03.06.2017): https://yadi.sk/d/MIo_CrLz3Jo8W8
Win32 (03.06.2017): https://yadi.sk/d/XmVGnWgm3Jo8GE

After replacement start wallet again. Wait for full sync before import!  
Advanced --> Accounts --> Import wallet
 1st field: paste Seed you saved in file before
 2nd field: type
clear keys
 And press Import Seed
 Then check if your old account is restored. If you use more that one account, add others in Advanced --> Accounts --> Create account

What error? If error deserializing config after downgrade, backup %LOCALAPPDATA%\RaiBlocks and then delete or rename config.json from there.