We missed the "free" upgrade window (some people have managed to upgrade without paying). I did catch the upgrade for my "banking machine" but when I picked up a used box for the living room, the license was dubious. I had other machines on line so I could download the ISO from Microsoft. I have some exposure and wanted a "legal" license key. Fortunately, these are available on line from places like KeyStoreOnline for $30. If you want to avoid the task of downloading and burning an ISO they also sell a bootable image on USB for $20.
Of course, you should backup your data. Windows has had a decent backup solution since Windows 7. Procedures are documented online to which I can add little so go hit Google or DuckDuckGo. N.B. Google changed there search rankings earlier this year to favor "authoritative" sources - https://www.wsws.org/en/articles/2017/08/08/goog-a08.html. Curiously this SEO censorship may drive traffic to the very sites they are trying to suppress.
If you want to bypass Microsoft altogether you can by a bootable USB stick of Mint 18 from https://www.bonanza.com/