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#11
You can find Windows 7 Home product key and installation DVD for $60 - $70 on amazon. You will boot the computer from the DVD and do a clean install of Windows 7 (select Install Now, then Custom Install, delete all the partitions on the hard drive, install Windows 7 to the resulting unallocated space). Then, later on when it gets all the updates that will take 2 - 3 days you will have to block the one for Windows 10.
This will completely erase EVERYTHING that came with the computer (except the Windows 8 product key will still be bios should you want to go back to Windows 8 but that won't affect Windows 7 at all.)