There is no "official" way to downgrade, unfortunately.
There is the "Flashtool" way, which is as easy as official, believe me.
Download Flashtool:
http://androxyde.github.com/
Download official ROM ftf file.
Follow this tutorial:
http://forum.xda-developers.com/showthread.php?t=928343
It might seem confusing at the start, but believe me, it's laughably easy once you're familiar with Flashtool.
