AT&T has made Windows 10 Mobile update available for all its eligible devices with the exception of Lumia 640 XL. But it seems Windows 10 Mobile Anniversary update is now available for Lumia 640 XL on AT&T.
This has been confirmed by an AT&T support forum discussion. As per the discussion in the thread, the Windows 10 Mobile update for Lumia 640 XL has become available starting October 27. Even a Reddit user has posted about the update availability. The release build for AT&T Lumia 640 XL is 10.0.14393.321.
Update summary Effective October 27, 2016, AT&T and Microsoft released a software update for the Microsoft Lumia 640 XL. You can download the update via Firmware Over The Air (FOTA). A Wi-Fi connection is required for the update.
You will need to install the Windows 10 Upgrade Advisor for getting Windows 10 Mobile update on your Lumia 640 XL.
Read our Windows 10 Mobile Anniversary update review and massive changelog + hands-on demo of changes vs Threshold by clicking on the links below.