Yesterday AT&T started rolling out the update to Android 5.0 Lollipop for the Samsung Galaxy S4. A bit odd was the fact that the newer Galaxy S5 hadn't yet received the Android version bump at that point, and usually older devices get updates later than their successors.
Regardless, AT&T is now doing right by the owners of Galaxy S5 units purchased from its stores. The Android 5.0 Lollipop update is now officially on the way to AT&T's Galaxy S5 as well.
As you can see from the screengrab to the left, it's a pretty hefty 1.4GB download, so you must use Wi-Fi to get it. That is, of course, if you've already been notified about its availability. If that hasn't happened yet, don't worry - it will within a few days, as the rollout is staged.
If you can't wait that much, you are able to manually check for the update by going to Settings > System > About device > Software update.
After applying the update you'll be running Android 5.0 Lollipop, build LRX21T.G900AUCU3BOC4. According to AT&T's notes for this release, nothing carrier-specific has been added. Every new thing you'll see after updating is part of Samsung's interpretation of Android Lollipop.