Just bought factory unlocked At&t galaxy s6 using t-mobile. How do I update to marshmallow?

Galaxy S6 running lolipop...factory unlocked At&t phone but using t-mobile service. How do I upgrade from lolipop to marshmallow. Tried "about phone" "software update" but it's for the AT&t server and is rejected. I've tried Smart Swith and it says software up to date 5.02.

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