There are actually 3 options. 1. Upgrade your existing Win8.1 to Windows 10, keeping all your installed programs and files and junk, clutter and registry bloat. 2. Upgrade Win8.1 to 10, have it activated then install Win10 as a clean installation from the ISO.
But the 3rd option, which I have done, is reformat and clean install of Win8.1 then upgrade to Windows 10, then reinstalled programs. So it's an upgrade, but a clean one and I still have Windows Photo Viewer (someone elsewhere on this forum is complaining a clean Win10 install doesn't include that).