i saw a thread about this but cant find it
For firmware 2.5:
1. Make sure you have 2 memory sticks.
2. download the 2.6 firmware upgrade and have it on teh 1st memory stick (PSP/GAME/Update/EBOOT.php) or whatever it is.
3. have 1.5 firmware on your second memory stick.
4. Start the 2.6 upgrade with the 'AC cord' disconnected
5. when the EULA (or somthing) message pops up and says "please plug in the AC cord" switch memory cards to the one with the 1.5 firmware on it and continue.
6. pray and see what happens
i give credit to the guy who came up with this...iono who that was soo yah.