This is just an update with the old post on PSP 3.90 M33 Custom Firmware Upgrade.

All you need to do is follow these simple steps:

1. Download Update File
    4.01m33-2 update 
    
2. Extract the contents of the file and put UPDATE folder in GAME Folder
    
3. Run the m33 updater in GAME, and wait for update to finish
   Make sure you have at least 75% battery level when you update/upgrade