Here is the official update guide for BackTrack R2.
Upgrading - BackTrack Linux
EDIT: This guide will work for upgrading from BackTrack 4 Final or BackTrack R1 to BackTrack R2
If you have problems with this please post them here in this thread.
EDIT: I did not write this guide. Muts did. So any and all credit goes to him. I just linked to it here for him.