You must have Win8.1 installed before the free Win10 upgrade will be offered to your computer via Windows Update.
No, you don't. I never installed 8.1. Three days ago I checked something on a Microsoft webpage via a link provided on this forum, which advised me that I had Windows 8 and that I was looking for something pertaining to Windows 10, did I want to upgrade to 10 now. I'm now running 10.