In the last month, both Sprint and Verizon have updated their Galaxy S6 and S6 edge units to Android 7.0 Nougat. These are some of the last phones to receive the 7.0 update, but hey, they're two-year old Samsung carrier devices. Now, AT&T is sending Nougat out to the Galaxy S6 (SM-G920A), Galaxy S6 edge (SM-G925A), and its exclusive Galaxy S6 active (SM-G890A).
Obviously, the main change here is Android 7.0 Nougat, which actually brings some pretty major changes on Samsung devices (new settings menu, updated multi-window implementation, general UI changes, etc.). AT&T also lists "other fixes and enhancements," and the pre-installed AT&T Address Book and MobiTV apps, which many probably considered bloatware, will be saying bye-bye.
The update weighs in at around 1.2GB for the standard S6, 1.3GB for the S6 edge, and somewhere in between there for the S6 active. The build number is NRD90M.GxxAUCU6EQCF for all three devices, and the April 2017 security patch is bundled in. The S6's, S6 edge's, and S6 active's update pages have all been updated to reflect this.
Let us know if your devices have been updated to Nougat yet. If your AT&T S6-series phone hasn't already received a notification, it should very soon.