It has already been reported that the Galaxy S6 and S6 Edge are getting updated to Android 5.1. Now it looks as though, they are not the only Samsung devices receiving an update. Having recently rolled out the Android 5.0 (Lollipop) update to LG's G2, America's second largest carrier, AT&T, has just announced that it is starting to do the same in a phased manner to Samsung's Galaxy S5 Active and Galaxy Alpha (SKU 6621A, SKU 6629A and SKU 6630A) with immediate effect. The Galaxy S5 Active was thus far on Android 4.4.4 (KitKat), while the Galaxy Alpha was on version 4.4.2.
The update to Android 5.0 brings along with it Google's Material Design with priority mode notification settings, Smart Lock, Screen pinning, device sharing, along with new battery saving features and many other enhanced UX (User Experience) features to both devices. The most notable under-the-hood change is the switch to the Android RunTime (ART) as the default runtime, opposed to the Dalvik Virtual Machine, or DVM. Given that the carrier is just starting to release the update, it might take some time for it to go live across all regions. Galaxy S5 Active owners on AT&T should expect the update file to be around 1.4GB in size whereas it's just a shade under 1GB on the Galaxy Alpha. The updates to both devices have only just started rolling out now, so it might take some time for you to receive any notification about it. You may however, check for it manually. To do so, you could head over to "Settings > About Device > Software Updates > Check for Updates".
According to an official release, the update will be available via Firmware Over The Air (FOTA) and AT&T is strictly mandating that the downloads only take place over Wi-Fi, so make sure you're connected to a high-speed Wi-Fi network before you start the fairly large update(s) and that, your phone has a full charge before beginning the update process because you can brick your phone if it loses power in the middle of a firmware update. For the Galaxy S5 Active, the built number for the updated software is LRX21T.G870AUCU1BOC5 and for the Galaxy Alpha (G850A), the build number is LRX22G.G850AUCU1BOC6.