Finally, Windows 10 mobile update available for Lumia 1520 devices on AT&T. If you are an AT&T Lumia 1520 user, this is your day to enjoy the official Windows 10 on your lovable Lumia 1520 device.
— Sathish Kumar S (@sathishk_sp) June 23, 2016
The update is now started rolling out. Use Upgrade advisor app to download the Windows 10 mobile update. As you already know, Windows 10 mobile brings many new features as well as huge improvements. Windows 10 update will boost up your Lumia 1520 experience on that big screen device.