Could you please elaborate the method ? In OSX-10 Sierra, I have installed Windows 10 using Bootcamp. I have updated Windows 10 to latest 1703 release also. How do I make a backup & restore this Windows partition ? Bootcamp accepts only an ISO image. Do
I have to repeat the whole process again to re-install Windows from scratch ?
What you want to do in this case (probably too late now) is make that bootcamp partition in macOS/OSX, but do not install Win10 to it. If you do, you won't be able to unselect "Format partition" option.
Create a boot USB drive using Media Creation Tool (see https://www.howtogeek.com/186775/how-to-download-windows-7-8-and-8.1-installation-media-legally/) and then boot to that (holding Option key). Then go to Troubleshoot -> System Image Restore and hook up your external drive that has the System Image you created in Windows 10 from the "Backup & Recovery (Windows 7)" part of the Control Panel. The "Format" option should be unselected for you and keep it that way.
So now you'll be able to restore from the image without having to format the whole drive. Hope this helps!