downgrade/upgrade

A long time ago I updated my PSP 1000 with 1.50 to 3.52 M33 which sort of ruined my PSP. Now after a few years(?) or so I want to start getting my PSP to work again with the latest 6.39 CFW which seems to work great for my friend. Except I am not sure how to get there step by step as I am sort...