Need Help using the Time Machine!

Yo!
Umm... im still new to this and all but im having trouble using dark_alex's time machine (i read the guide and all but it doesn't relate to what im doing). Well basically im tryin to downgrade one of my psp (5.00 m33-6) to an 3.90 cfw, b/c one of the game sav files won't boot and i don't want to start over my game. The main prob im having right now is how can i downgrade it to a 1.50 fw or any prior cfw. Also im not sure if you can downgrade to a 1.50 cuz one of my friends told me that. Sry if my english sucks and if anyone needs to clarify anything just post something thk!

btw i have a jigkick battery and mms but it will only load it to an 5.00 m33 cfw and i have 2 phats (5.00 m33-6 and 3.93 fw) and a slim (5.00 m33-6) a lot of 2G ms and 1 4G ms. Also it would be helpful if you can give me the links. Arigato Gozaimasu (Thank You Very Much! in Japanese)