Recently I made a clean install of Windows 10. I was currently on Windows 8.1. I did not wait for that free update but instead went for that clean install and ruined my free Windows update and now I am lacking lots of features because of it. Problem now is I want to revert back to Windows 8.1 but I don't know my product key. I can't do a revert because for some reason my windows.old file is missing and I can't find it. Can someone please help me on how will I be able to retrieve/extract my previous product key? I believe mine is in BIOS as I don't have any DVD or any box for the OS when I bought my laptop.