Finally, after waiting for long AT&T released Windows 10 mobile official update for Lumia 640 users. Its been some months, Lumia 640 users under AT&T waiting for the update. Finally the dream fulfilled. As per the reports by users in AT&T forum,Windows 10 mobile update is now rolling out to Lumia 640 devices.
— Dona Sarkar (@donasarkar) June 3, 2016
You need to use upgrade adviser app to receive the update. If you are using an AT&T Lumia 640, you can download the official Windows 10 now on your handsets.