Back in May I posted that Samsung had released their Android Marshmallow update for the Galaxy S5. AT&T, it seems, just now got around to releasing it. During the same week that Google released Nougat, the US carrier released Marshmallow for the S5 and S5 Active on their network. That’s a full three months after Samsung released and just shy of three months since Verizon released it for S5’s on their network.
Come on AT&T, get with the program.
If you have a Galaxy S5 or S5 Active, you should be seeing the update come to you via an OTA update. The update is about 940MB so you will certainly want to be on Wi-Fi to download it. Once you have it downloaded, the install including reboots should take about 30 minutes. So all in, likely 45 minute to an hour to upgrade completely.