AT&T has started rolling out the Android 5.0 Lollipop update for the metal-clad Samsung Galaxy Alpha, and the durable Samsung Galaxy S5 Active on its network. The updates are being rolled out OTA (over-the-air) and in stages, so it may be a few days before it is available for your handset.
Once you do receive the OTA notification, be prepared for a fairly large update process, with the files measuring 1.4GB for the Galaxy S5 Active and 1GB for the Galaxy Alpha. Due to its large size, it is recommended that users connect to WiFi to avoid unnecessary data charges. It is also recommended that you ensure you battery is appropriately (at least 50%) charged during the update process.
Once you have completed the update, your handsets will be running Android 5.0.2 with a new Material Design inspired user interface, Lock Screen notifications and the 64-bit ART compiler which should help apps open faster. Also included is Project Volta for enhanced battery life, the new "Recent Apps" multitasking menu, easier access to Quick Settings, and more.
source - Android Central