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#11
Boot it from Kyhi's rescue disk and run showkey plus from that. That will tell you what the product key is stored in bios and what edition of Windows it is for:
Windows 10 Recovery Tools - Bootable Rescue Disk - Windows 10 Forums
If you start with a clean install for the same version of Windows 10 as the Windows 8 product key in bios is for, it won't even ask you to enter a product key - it will just use the one in bios. For a clean install you can mount a Windows 10 ISO or insert a Windows 10 USB or DVD with Kyhi's Recovery running and just run setup.exe from it.
Or see my quoted note at the bottom of the link for Kyhi's recovery drive opening post.