Not an expert on this, just getting into it all again myself, but from what I have gathered so far, if its already running 1.50 firmware, just download the 3.50 m33 custom firmware, and then update from there to the 3.90 m33-2 firmware. I read that you can't go straight from 1.5 to 3.9, gotta go 1.5->3.50->3.9
I believe thats how it works.