Ok I found where you can find all the custom firmwares. You need to be at
- Installation requires a PSP with 3.52 M33-3 or higher. In the future installation using
panddra battery will be released.

to get to 3.80 m33-5

To get 3.52 M33-3 or higher go http://forums.maxconsole.net/showthread.php?t=75846

I have no idea the install path from your firmware so you might need to check the rar for instructions.

From 3.52 m33-3 you can go to 3.80 m33-5

I had a slim so I was already on 3.7 so the slim got Custom Firmware'd to 3.71 m33 to 3.80 m33-5