EDIT: Make sure your PSP is fully charged and plugged into AC power before you start.
Backup your game saves first, just to be safe (Copy and paste the \PSP\SAVEDATA folder from you psp to your pc somewhere).
You need 3 things, 2 to trick the psp into thinking its a lower firmware: eloader and the downgrader, and the psp firmware version 1.5 placed in \PSP\GAME\UPDATE\eboot.pbp (the 1.50 pbp eboot file) to 'update' to 1.5 once you've used the downgrader.
If your running windows just use the 'installer version' of the eloader from here: http://www.dcemu.co.uk/vbulletin/showthread.php?t=36627
At the 'installation options' screen either tick the 'gta version' checkbox or choose the correct option for you from the dropdown box.
Then, copied/pasted from the eloader readme.txt:
'Running eLoader : via GAME mode
-------------------------------
- Load up the GTA game from the UMD.
- Open the menu with 'START', then select 'Load Game'.
- Select the eLoader icon from the list of saved games.
Confirm that you want to discard current progress.
- The eLoader should start after a few seconds.
- If instead, GTA loads a normal savegame, then you have the
patched version of GTA.'
You should be at the eloader menu and have the downgrader as an option. Read through the downgraders readme and off you go.
Once its done its work you just need to go to the game menu and select the 1.5 update.
From Kaiser's 2.0 downgrader guide:
'If so all is well, goto the game menu and Launch the PSP Update and simply
follow the onscreen instructions to update your psp. The update will get to
99% and Fail, No cause for concern this is still correct. The psp will now
do a faily length restart just let it play out. Now one of two things will
happen either the PSP will restart or it will turn off. If the PSP does not turn off automatically you can perform a hard reste to turn it off. After the LED goes off and the psp has turned off.. Wait 10-15 Seconds and power the PSP back on. You now have a 1.50 PSP, Congratulations.'
For me the psp just froze at 99%, I left if for a cpl of mins to make sure it had finished, held the power switch up for 5 secs to hard power-off, turned it back on and have been a happy 1.5 user ever since.
Hope that helps, try to search the forums for answers before you ask questions where possible.
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