Just days after Verizon started updating the Nokia Lumia Icon to Windows 10 Mobile, the update has begun hitting Lumia 1520 units on AT&T's network, bringing features like universal apps, slow-mo video capture, and HDR. @Daniel_Rubino My ATT Lumia 1520!! pic.twitter.com/he9hU9LHZS— Sathish Kumar S (@sathishk_sp) June 23, 2016 Given the update has just started rolling out, it may take sometime before it hits your device. Meanwhile, if you feel impatient, you can manually check for the update by heading to your handset's Settings menu. Via
Source: http://www.gsmarena.com/windows_10_update_starts_hitting_at_t_lumia_1520_as_well-news-19037.php
