AT&T Galaxy S6 unlocked for Tmobile not updating

[AC Question]

Hi all, I received a S6 and it is on firmware 5.1.1, I'm trying to update to marshmallow but everytime I go to "Software update", it just says "software is currently up to date" I have a SM-G920A. I tried using smart switch and it doesn't show the update button. There are no firmware updates for my phone on sammobile and I can't find a solution for this. Someone help me please

 

[anon(632115)]

Either use a valid AT&T sim or take to a Best Buy. Those are the only options