Here Go there and scroll down to 3.51M33 and click on release.
You'll need something like WinRAR or whatever you use to unzip files.
Then:
1 - If you are under official 1.50 PSP firmware, copy kxploitpatcher and kxploitpatcher% folders from "1.50only/" of this download to your "ms0:/PSP/GAME/" and use it.
2 - Once done it, or if you are under any Special/Open Edition, copy M33CREATOR folder to your "ms0:/PSP/GAME" (1.50fw) or "ms0:/PSP/GAME150" (SE/OE)
3 - Download official 3.51 eboot, rename it to 351.PBP and place it on "ms0:/PSP/GAME/M33CREATOR/" or "ms0:/PSP/GAME150/M33CREATOR/"
4 - Download official 1.50 eboot, rename it to 150.PBP and place it on "ms0:/PSP/GAME/M33CREATOR/" or "ms0:/PSP/GAME150/M33CREATOR/"
5 - Go to Games/Memory Stick on your PSP, and execute "3.51 M33 CREATOR". Once finished, it will go back to the XMB.
6 - Go again to Games/Memory Stick on your PSP, and execute "3.51 M33UPDATE". You'll have to press X to agree to the installation.
7 - When finished, press X to shut down and reboot manually.
8 - Enjoy your 3.51 M33 ;)
By the way, M33 wrote the above^^
All credit goes to them.
Thanks M33 =]
