The entire Galaxy S6 line from AT&T should now have Nougat, thanks to updates that started for the S6, S6 Edge, and S6 Active last night.
Each update includes the April 1 security patch from Google, along with software versions G920AUCU6EQCF (Galaxy S6), G925AUCU6EQCF (Galaxy S6 Edge), and G890AUCU6DQD1 (Galaxy S6 Active). They are also all Android 7.0, not the newer 7.1 that has been out for almost as long. In terms of size, you are looking at 1.2GB at least, so make sure you have plenty of space on your phone and a whole bunch of battery. This is a big one.
The updates should bring better split screen or multi window support, quick replies to notifications within your notification panel, new battery management features, new effects screen in the camera, performance modes, an updated keyboard, and more.
To check for updates, head into Settings>About device>Software update>Check for updates.