S5 won't update

[eek2002]

The last 3 build letters are OF3....G900A AT&T unlocked using cricket wireless it still on 5.0 version want the new Marshmallow 6.0...I need help of doing this, I try putting AT&T Sim card it didn't work just says it's up to date. I think it need to be hook to a computer and done that way. Samsung Smart Switch even said it's up to date it can't be. Please help me with this I'm new at this kind of thing.

Thank You