If you want to downgrade so you can get custom firmware, you don't need to. Use the link at the bottom of my signature to get 3.40 OE-A custom firmware.
If you just want to downgrade to 1.5, the easiest, not only, but easiest way to do it is to update to 3.50 here:
Firmware 3.50 now available on Network Update - PSP Fanboy
Then downgrade using this downgrader here:
Article Detail - PlayStation Portable News - PSP Updates
Note: You need a UMD copy of Lumines for the downgrader to work.
I suggest the first option, but either will work.
If I helped you, please give me karma!