Verizon was the first carrier in the US to release Android Marshmallow to the Galaxy Tab S2. Unlocked variant of the Galaxy Tab S2 was updated to Android 6.0.1 a few weeks after that. Now, AT&T has started seeding the Android 6.0.1 update for the Galaxy Tab S2 on its network.
The Galaxy Tab S2 (SM-817A) has started receiving Android Marshmallow update (with firmware number T817AUCU2BPF20), which also brings updated UI design, multi-user support, Google Now on Tap, granular App Permissions management, Doze mode, and various other little features.
The update, which has a file size of 1074MB, also brings AT&T's Number Sync feature that gives users an option to use the tablet to make and receive voice and video calls using your existing number. It works even when your smartphone isn't present with you. The update is rolling out over the air in the US.
If your device hasn't received the update, head to Settings » About device » Software update » Check for updates.