How to reflash a PSP to non-custom firmware?

Okay, so my brother had a friend of his (who is now unavailable) flash his old PSP-1000 to non-official firmware a while ago, but no longer wants the handheld anymore and is willing to just let me take it. Now the thing is I can't upgrade this PSP's software regularly through the operating system for some reason and to be honest I don't care for non-official firmware's features at all (I would just like the non-hassle of official firmware). So, how do I reflash to the official software? I believe the console is currently running OS 3.52 m33 or something like that. Thanks!