Verizon updated the Galaxy Tab S2 to Android 6.0.1 last month, and AT&T did the same a couple of days ago. It's now time for T-Mobile to do the same. T-Mobile has finally started updating the Galaxy Tab S2 on its network to Android Marshmallow in the US.
The LTE variant of the Galaxy Tab S2 (SM-T817T) on T-Mobile's network has begun receiving Android 6.0.1 through a firmware-over-the-air (FOTA) update. The 1022MB update bumps the Galaxy Tab S2's firmware up to T817TUVU2BPF1 and brings new features such as App Permissions, Doze, Google Now on Tap, multi-user mode, Nexus Imprint compatibility for the fingerprint sensor, and an updated TouchWiz UI among others.
If you haven't received the update, head on to Settings » About device » Software update» Check for updates.