Windows 10 Roll Out for AT&T Customers:Be sure to check with the Update Advisor to see if it's ready for yours. Owners of the Lumia 640 have also been sharing similar reports on our own forum, so be sure to join in the conversation if you're yet to receive the update or have some handy advice to get others on the Windows 10 Mobile train.
Steps to Upgrade Windows 10 Roll Out by AT&T in Lumia 640:
- Go to Upgrade Advisor and check if you have any updates available for your mobile.
- If you see the screenshot as the same as above click next.
- Go to settings > Update+backup > phone update in Lumia 640.
- It will then download the updates and install them. The update is going to take a little more time than you think, So don't worry about that.
- Now you are done and when the update is done it will send a notification that your upgrade is ready and boom. You have windows 10 installed in your Lumia 640.