After Sprint and Verizon, AT&T is sending Nougat to Galaxy S6 (SM-G920A), Galaxy S6 edge (SM-G925A), and its exclusive Galaxy S6 active (SM-G890A). However the size is different for each device. 1.2 GB for S6 while 1.3 GB for S6 edge. Also the update contain security patch April 2017.
Beside the usual Android 7.0 Nougat update which brings huge changes like new settings menu, updated multi-window implementation; it also brings some fixes and improvement with pre-installed AT&T Address Book and MobiTV apps.
SOURCE – ANDROIDPOLICE