Backup Your Mac
While the macOS High Sierra update is relatively safe, you are installing a new version of macOS on your computer and it's irresponsible to do so without a current backup.
You need to make sure your photos, documents, apps and other info is safe and secure in case something goes wrong while you are upgrading. This is a relatively simple process on most Macs. If you have an external hard drive plugged in you can make a Time Machine Backup of your Mac that keeps everything safe.
If you already have this setup, you should go to Settings -> Time Machine and make sure you have a recent backup on that drive, especially if you use a laptop that you don't always plug in to a hard drive. Here's more on how to use a Time Machine to backup your Mac.