How should I go about backing up Windows 7 to upgrade to Windows 10?

You can use one of the free online storage sites to back your files up to.

Or you can copy them to an external hard drive, DVDs, thumb drives, etc. It doesn't matter. It's just whatever you happen to have on hand to use, or what you want to spend money on to buy. If it were me, I'd get an external HD. Just back up your important files such as music, videos, pictures, Word Files, Excel files, etc etc

Keep in mind that backing up is just a precaution, not a necessity, because your files ARE NOT HARMED and carry fully forward to Win 10. The only time they would come up missing is if you have a catastrophic failure during upgrade (such as a power outage). But the files are perfectly fine otherwise. I did not back anything up when I did my upgrade.

The safest way to do this is to clone (not copy) your existing HD onto a second HD. If the upgrade fails or you want to restore to Win 7, just swap drives.

There are 2 ways to do this. Put the 2nd HD in a USB 3.0 enclosure and clone vis USB or in a desktop, install the 2nd drive in the PC. If the 2nd HD is not formatted before cloning, Windows will not see it but Easeus will see it and allow you to clone.

There is free software to do this. I have used Easeus ToDo http://www.easeus.com/backup-software/tb-free.html...

Don't worry about Windows.

But saving / copying your own files to a backup disk / USB hard drive is ALWAYS a good idea!

save your documents to DVD or to an extra hard drive or computer