I need help Modding My PSP 2001 series

its at 3.9 Firmware
and i need help downgrading and then
putting on the custom firmwarehelp would be great

pandora battery. it involves removing a solder from your battery. google the term, i have no experiense with the psp slim and litejust in case you do, tell me how the n64 emu runs playing super smash bros, will you :D