AT&T has finally started to roll out the update for Windows 10 Mobile to its users. The update is rolling out and if you own a Lumia 640XL via Carrier AT&T then you should be able to get the update to your device.
You need to install the Upgrade Advisor app to check if you are having any update available, if yes then you need to head to Settings>Phone Update>check for updates.
One of the users of Lumia 640XL with AT&T has posted on Reddit in regards to the availability of the Windows 10 Mobile update.Once you update your 640XL you will be on the RS1 Build 14393.321.
IF any of our users is using the Lumia 640XL with AT&T then you can get the update via Upgrade Advisor, You can download the same via below link.