This tutorial shows backing up your site with a premium plugin called Backup Buddy. If you use a different backup system that will work too, but you should always have a backup before updating. Just in case.
Affiliate notice: I am an iThemes affiliate and will receive a commission if you purchase Backup Buddy using my link. That said, I don't recommend products unless I also use them myself, and I use BackupBuddy on every website I build.