HOW DO I DOWNGRADE MY PSP FROM 4.01 M33-2 TO 1.50?

ANSWERS ASAP PLEASE! how do i get 1.50 on my psp so i can upgrade to 3.71 OE-B to play Oddworld on my psp? i have a PHAT pandora battery a MMS and a normal one. can u give me some links that worked for u or almost did? I WILL CHOOSE BEST ANSWER SO ANSWER PLEASE!