Samsung and its carrier partners can take a long time to get big system updates rolled out to older devices, but at least it happens. Some OEMs would have abandoned a phone from 2015 by now, but T-Mobile is rolling out a Nougat update to the Galaxy S6 and S6 Edge, which was promised a few days ago. Bonus: the Tab S2 on T-Mobile is getting the update, too.
T-Mobile has updated its software docs for all three devices (GS6, GS6 Edge, Tab S2), which doesn't offer much in the way of detail. The changelog for all three updates just lists Android 7.0. That means some UI tweaks, standard Android multi-window mode, improved doze mode, and more.
Head to the update menu in your device settings if the OTA has not already appeared. It's likely a staged rollout, so it might take a day or two before it shows up.