Don't have a lumia 640 myself but you should be able to use Microsofts new over the cable update tool with your phone connected to pc over a usb cable. The tool can be downloaded from the link below.
https://forums.windowscentral.com/e...ad%2Fdetails.aspx%3Fid%3D56120&token=kywEr08V
As ever make sure phone is fully charged and that you have enough free storage on your phone. Also check that text massages and contacts are synced with your account before update. When on wm10 you shouldn't restore an old backup to avoid problems, just login with your account. Check for updates and update to latest wm10 build and do a hard reset before you reinstall your apps.
Maybe some lumia 640 owners who tried this can comment on the details?