You need to use a recovery pack (google it). Place the files from the recovery pack in a folder called RECOVERY in the GAME folder of your memory stick, then put the 1.5 eboot in the same folder, but rename it UPDATE.PBP. You should then be able to downgrade from the recovery console. To use the recovery console you have to power off by holding the power button up for a few seconds, then hold 'R' while powering up again.