AT&T version of Samsung Galaxy S5 receives Android 5.1.1 update27
The update weighs in at around 610MB, and you should be on a Wi-Fi network when downloading and installing it. In addition, you should make sure that your battery is charged to at least 50% before you begin the process of updating your phone.
If you are receiving the update OTA (as opposed to syncing with your PC and using Samsung Kies), keep in mind that it is sent out on a staggered basis. That means it might take a few days before it finally reaches your handset. If you want to try to manually get the update out, go to Settings > About Device and check to see if it has arrived.
This should not be the last update for the phone. Earlier this month, a list of Samsung models allegedly tapped to receive Android 6.0 Marshmallow included the Samsung Galaxy S5.
source: AT&T via AndroidCentral