You can downgrade your PSP, but you will need a copy of the original release (has a firmware 2.0 update on the UMD) GTA: Liberty City Stories.
The only way to downgrade a PSP with firmware 2.80 or above (without a modchip) is through an exploit in this game, the later (patched) version will not work.
If you can find a copy, you can use this downgrader.
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