AT&T is now finally in the process of rolling out the update to Android 5.1.1 Lollipop for its version of the Galaxy S5, Samsung's flagship from 2014. It's quite late, of course, seeing as how Google's already released Android 6.0 Marshmallow to its Nexus devices, but at least owners of the AT&T Galaxy S5 now have something to look forward to.
Aside from bumping up the Android version to 5.1.1, the update also adds Android for Work, a few security updates, as well as a fix to Ultra Power Saving Mode for phones using on-device encryption.
As this is an over-the-air rollout, it may be a few days before you receive the notification that the update is available for your handset. If you're impatient, you can always manually check for the new software by going to Settings > About device > Software update.
Do note that you must use Wi-Fi to download the new release. The file size is around 609MB. After you've applied the update you'll be on build version LMY47X.G900AUCU4COI5.