If you are still feeling brave, read through a couple of times and do a 'dummy run' so you're comfortable with the menu thing but don't actually do the disable in step 3 until you're sure you know exactly what to expect. You are ultimately responsible for whatever occurs, not me - I am simply relaying info from the other site as a courtesy because this happened to work for me. FInally, before you commit yourself to the following be sure that you do so willingly. Go to the PSPMOD site linked above for further assistance.

If your PSP is not a Slim model and the firmware is not as shown above then do not go any further. OK, if you have a PSP Slim and either wireless network access to the internet or a copy of the current official firmware saved to some storage medium When you check out the system information, if your custom firmware (CFW) is version 3.80 M33 or greater (in my case it was 4.01), then the following edited info from PSPMOD applies. Even just one person because then, it was worth writing. I hope this instructable was useful to anyone. keep tapping 'O' and you will be prompted with the default setup as you did when you first got your psp. 11.When powering back on, you will see a blue error screen with lots of words in different languages on it. Do this by holding down the power button. When done, manually restart the psp by completely shutting down again. you will be prompted with a flash screen (recovery mode ,main menu) 9.Select the option that reads Run program at PSP/GAME/RECOVERY/EBOOT.PBP 10.Follow the options as is. 7.Completely shut down your psp by holding down the power button or removing the battery (make sure you are NOT going into sleep mode) 8.Press and hold down the right shoulder button while powering on the psp. 5.Put the recovery folder into PSP/GAME/ 6.Exit USB mode. 3.If you have not downloaded the file from the 'what you need step', download it and extract it using win rar.(oh yeah.install winrar first) =P 4.Connect your psp by going into USB mode. Back up all your data on your memory stick as you will be formatting it.

How to update psp cfw without battery