you can use WAB version changer.
this will make your psp 2.00, 2.01 or 2.50 while your running firmware 1.5
note that this will not work with all games that require firmware update because the new firmware supports more file formats (like video avi and others)
or you can use mhp's new firmware launcher that updates your psp and you can still play homebrew as if you were running firmware 1.5 (its a custom firmware, sort of)