Along with the Galaxy S5 Active, AT&T has also pushed the Android Marshmallow update to the Galaxy S5 (SM-G900A) this week. The carrier has released this update to the Galaxy S5 units on its network three months after its competitors At-Mobile and Verizon did it in the US.
The update carries a build number MMB29M.G900AUCU4DPH2 and updates the Galaxy S5 to Android 6.0. The update is being pushed over the air and has a file size of anywhere between 938MB to 955MB. AT&T requires you to download the update only through Wi-Fi. You can check for the update on your device by navigating to Settings » About device » Software update menu and hitting the Check now button.
Changelog:
Android OS 6.0 (Marshmallow)
Wi-Fi Calling
Video Calling
Android device security enhancements