Your in the wrong forum, there is a PSP section below this one.
Anyways, I'll answer your question...
At this time, there is nothing publicly released that allows you to downgrade your firmware. Team Xecuter has been able to do it though, but they are not going to release it, as it requires a programmer of sorts I think, and they said that WAB(the creator of the ISO loaders from the MS stick), and other groups, are going to release a way to downgrade your firmware through software.