If you got the SE-B Full Installer:
1)Plug in a USB cable to the PSP
2)Goto USB Mode
3)Run SE-B Full Installer on the PC and select Install Recovery mode for returning to 1.5
4)Wait until that is finished, then turn off the PSP.
5)Turn on the PSP and hold the R as it powers on
6)You will be in Recovery Mode. There should be an option to: "Run program at PSP\Recovery\Eboot.pbp" - select that
7)Follow the instructions on screen and after a few minutes you should be back to 1.5