Upgrade your macOS
Before upgrading your Mac, it is very important to make sure it is running well and that it is freshly backed-up. You may think your Mac is running well, but it may have some problems that are not evident to you. A restart usually fixes this problems.
Simply go to the Apple menu in the upper left of your screen and choose Restart.
Wait for it to restart fully back to your Desktop
Now, let’s backup your Mac (make of a copy of its apps, settings and your content - pictures, videos, emails, texts, etc.) to somewhere safe. This is the most important part of updating so you can get back to where you are if something goes wrong. I recommend you use Time Machine to do this. See my blog about backing up your Mac if it is not already set up to do so.
1. Go to the Time Machine icon in your menu bar near the clock and choose "Backup Now".
2. Click the Time Machine icon again to watch its progress until it is done backing up.
Finally, go back to the Apple menu and choose App Store. Once open, search in the upper right for "High Sierra". When it finds it, click it and choose the Get or Download button. Once downloaded, the Installer app will start instructing you to install the upgrade. Follow the instructions. That's it!