Help: best way to back up Windows

Hi all,

Got my Delta 3 a few months ago from the Kickstarter campaign and got one that came with Windows 10 Pro Education. Curiously, I didn’t have the license key sticker that they mention in their docs. Planning on doing some tinkering in Linux but do eventually want to go back to Windows.

What’s the best way to backup/restore Windows with the key intact? The official docs seem to say that I need to furnish the key that came in the box (there was no key). Already tried a dd clone of the emmc to an external drive and boot from there but it didn’t seem to work.

Suggestions please!