Download the lastest firmware from the link alone posted, then plug the PSP into your computer and put the update file (I think it's called "EBOOT.PBP") in the /PSP/GAME/UPDATE/ directory of your PSP's memory stick (create it if it doesn't already exist), unplug the PSP, then run the updater through the Game > Memory Stick menu option. You'll need to have at least 70% battery.
Oh I remember my days of PSP custom firmwares, that was fun...