I made a walkthrough on how to downgrade any firmware 1.51-2.6 to 1.50. This is the 2.01 section.

And you can get FW 2.50 from here.

v2.01

YOU MUST HAVE AT LEAST 80MB FREE ON YOUR MEMORY STICK!!! Even though the folder is 30MB, and the update 17MB, the downgrader creates about 20-30MB of other files on your Memory Stick.

First load the files onto your PSP.

Then, run the 2.50 update from the Game menu on your PSP. This will take a few minutes, and you need the charger in. When that is done, start up GTA, and load the savegame called eLoader Bock. When it has loaded, press X to run DOWNDATER TEST. The orange memory stick light will blink for a few minutes, then it will reboot. An error mesage will appear, in different languages, saying setting information is corrupted. Press O to fix it. Your PSP will reboot, and you will have to run the setup like when you first bought your PSP. When you finish, there will be another error saying Disc Could not Be Read. Take out your battery pack, and leave it out for several minutes. Put it in, and UMDs should work again. If not, try taking out your battery again. You will be running version 1.50.

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