Here is a step by step guide for installing 3.52 M33. Please note that you must be running 1.50, or any of the OE series firmware to begin this installation.
First grab Custom Firmware 3.52 M33 For PSP
and extract it.
1. If you are running firmware 1.5, first go inside the "1.50only" folder and copy both kxploitpatcher and kxploitpatcher% to X:/PSP/GAME and then execute the program through the game menu on the PSP.
2. Copy the M33CREATOR folder to X:/PSP/GAME if you are in 1.50, or to X:/PSP/GAME150 if you are running OE.
3. Now download the 1.50 and 3.52 updates which you can grab below: 1.50 Update 3.52 Update
4. Place the two updates you downloaded in the M33CREATOR folder.
5. Navigate to the game menu on your PSP, and execute "3.52 M33 CREATOR."
6. Once finished, navigate to the game menu again and execute "3.52 M33UPDATE."
7. When finished, follow the on-screen prompt to reboot. That's it! You've now upgraded to 3.52 M33.