1. Download PSPident:
http://dl.qj.net/PSPident-v0.4/pg/49/fid/26999/catid/151
2. Put it on your PSP in the folder PSP/Games
3. Navigate to Games on your Psp and start it.Write down your Motherboard if you can't remember it.
4. Check if your motherboard is supported here:
http://pspslimhacks.com/pspident-04-find-out-what-psp-motherboard-you-have/hackablepsps-1/
If not both is supported you have to try something else. If both are red then there is no chance to hack it.
5. Download the firmware 5.00:
http://downloads.pspspot.net/firmware/762-5-00-official-psp-firmware.html
and 5.03:
http://www.pchome.de/download/sony-psp-update_7038.html