Hi,
Thank you for posting your query on Microsoft Community. We are sorry for the inconvenience caused.
The release of Windows 10 for Lumia 640 mobiles have not yet officially announced.I suggest you top download the Windows Insider app and when your phone is supported you'll be able to sign in with the same Microsoft account you
used to join the Windows Insider Program.
Please refer the article on “how to install Windows 10 on Microsoft Lumia 640”
http://windows.microsoft.com/en-us/windows/preview-supported-phones
Please feel free to contact us, if you need any further help we would be glad to assist you.