The Nokia Lumia 1520 on the AT&T network in the US is receiving a new software update which brings version 1028.3534.1347.2001 to the device. The update does not include any new features, but it does reportedly bring general performance and usability improvements.
The update is being pushed OTA (over-the-air) and you will receive a notification when it is available for your device. You can also manually check for the update by heading to Settings > Phone Update. YOu can also watch the following video for guidance on updating your Lumia 1520.
source - WMPowerUser