Need to downgrade to original firmware

Hi. This is my first time on the boards, but I have a problem I need help with. I bought a couple of new games yesterday, and they both needed upgrades to play, but when I attempted to upgrade, I received this message: The game will not start. DADADADADADA.
I took it to gamestop, and they set it to the default settings, but nothing worked. We discovered that my PSP has been modified, but I didn't modify it because I don't know how to modify it. I think my son must've modified it without my knowing. It has version 3.52 M33, nickname P172, and it has a Mac address of 00:16:CE:89:DC:86, but I don't even own a Mac! I need to downgrade back to the original firmware that my PSP came with, whatever that is. They said I have a PSP 1000, and I am the original owner. It still plays all my old games and my movies--I just can't buy new games to play, because they won't upgrade. I appreciate any help I can get on this matter. I'd like to be able to purchase and play some new games for a change.

MAC address = Machine Access Code address. it's found on every ethernet card, wi-fi device, and modem.

and you can upgrade to latest m33 (IIRC that's 5.00 m33), then to 5.50 GEN-D3.

and even if you could downgrade to the original stock firmware the PSP came with, your games wouldn't work anyway, as they all had to be at a certain firmware or higher in order to play.