Well you'll need to use the 'Pandora's Battery and Magic Memory Stick' Method.
This is basically a modded Battery and Memory stick that when put together, enable you to downgrade your OFW to either 1.50 or a high CFW. On the front page is the most up to date version and I used it here recently and it works great.
However, in order to make this 'Pandora's battery and Magic Memory Stick' you need a psp that can run homebrew since the pandora installer is a homebrew app. So basically you need a psp that has CFW on it in order to make the battery.
Do you happen to know anyone with CFW or a very low firmware psp?
Well if you don't, you can either buy yourself a set off ebay or something, or you can use
This Tutorial to make your own Battery w/o CFW by opening it up and soldering a pin. And you can use your computer to make the Memory Stick files.