This is not currently possible yet! If you buy a modchip then you can but that's if you know how to solder.

__________________

Firmware 2.5 and above will never be downgradable EVER!!!!!! Just sell your 2.5 and get a 1.5 or get the Undiluted Platinum modchip and get someone to solder it for you.

__________________

there might be one like couple of years untill then just get a new psp or keep yor 2.5 or even get the undiluted platinum

__________________

There will never be one in any number of years 2.50+ is impossible to downgrade, IMPOSSIBLE!!!!!! And always will be. You can't replace the index.dat on any firmware above 2.01, 2.01 is the only downgradable firmware apart from 2.0 (1.51 and 1.52 are but nobody will want to downgrade those firmwares because their not used by anyone) so just get the mod-chip or sell your 2.5 and buy a 1.5. You could see if anyone you know has a 2.0 and ask to swap with them (maybe offer them the psp and some money) then downgrade it.

__________________

or trade your psp in and just get a new one for like 50 dollars

__________________