need to find Windows credentials after overwriting Win 8

I am unsure how to proceed. About 3 years ago, I bought a HP Pavilion (AMD Quad-core) desktop from Woot for my wife. She hated Win 8, so we put Ubuntu 12.04 (with Gnome flash-back [fall-back?]) on it and she was/is a happy camper.

It is time to upgrade to 16.04 and I hope I can get that done without posting here, but before that, there is an "opportunity" which expires in a few days (July 31) and that is to upgrade the Win 8 to Win 10 (which I am OK with, but not in preference to Ubuntu). I would like to upgrade her computer to 10 just to have it if needed, but there is no current copy of Win 8 on the machine. I do not see a Win sticker on it either. Have no idea where the documentation (if there was any) is that might have come with the box.

Any ideas on how to credential the computer for the upgrade? Thanks.

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