Kind of clueless here on how to do the free upgrade.
Wife won an Asus T100T tablet from a work party, the device came with Windows 8.1 installed.
I am not sure how to upgrade it to Windows 10. It seems it wants a product key but the tablet never came with any documentation or paperwork and I cannot find anything on any label other than the serial number on bottom of the tablet.
I’m not sure you need to enter a product key when you are doing an upgrade like you would be. I had a similar computer that came with Win 8.1 (with Bing) that had no product key. I let Microsoft do the upgrade, and it never asked for a product key.
Another computer I also upgraded. In that case I had downloaded the ISO image for the upgrades, burned it to a disk, and ran it from within the Win 7 that was on the computer. Again, no asking for a product key.
In both cases, the upgrade was done from within the running OS that was on the computer. I think if you do a fresh install from the CD — not as an upgrade — then you would need a product key. But, somewhere on the Microsoft site where I had downloaded that ISO, they were supplying a generic product key should you need it.
For me, I rolled both those computers back to the original OS — which you can do within the first 30 days. But, you may find Win 10 more to your liking than I did.
And, there are some utilities on the web that are supposed to pry out a product key from a running OS. Offhand I think one might be JellyBean, or something like that. I guess that would be another approach you might take.