If you have GTA UMD you should download this downgrade Easy 2.50/2.60 Full Downgrade
If you don´t have GTA UMD you should update to 2.71 by donwloading this Update and copy it to PSP/GAME/UPDATE, now update, when you have updated, download this Downgrade.
When you have the file, on your pc, open it, and you should do exactly what the guy says and you have a 1.50 psp

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