Light,
What firmware version are you using? Whether or not you can even do this depends on your firmware. You'll need to use a downgrader, and unfortunately, some versions do not have one available. It usually takes anywhere from a week to three months to release one. I was stuck at like 3.-something for a while. Let me know, and I'll help you from there. But just a word of caution, if you decide to do this, and it bricks, you'll be out a PSP. You have to re-write your flash memory then install a custom version. So many things can go wrong. If you mess up, expect to be creating a Pandora's battery and a "recovery stick" for your machine.
Just reply with the version, and we'll go from there.
Thanks