Apple makes it very easy to back up your Mac using Time Machine, so there is no excuse not to create a Time Machine backup before installing macOS High Sierra.
Follow these steps to back up your Mac using Time Machine.
• Connect an external hard drive to your Mac. Choose a new hard drive, or one that you don't mind erasing.
• An alert may appear asking if you want to use the drive to backup your Mac. If so, Use as Backup Disk.
• If not, open System Preferences > Time Machine and click Select Backup Disk. Choose the external drive.
• Switch Time Machine to On.
• A progress bar will appear in the Time Machine system preference pane. Wait for the Time Machine backup to complete before continuing with the macOS High Sierra installation.
Follow these instructions to install the macOS High Sierra update on your Mac.
• Once you've backed up your Mac, launch the App Store (located in /Applications) and search for macOS High Sierra.
• Click Download to initiate the download, and fill out your Apple ID information if prompted. Download progress will appear in your Purchases tab.
• Once the download has finished, you'll see a macOS High Sierra installer launch. Follow the on-screen instructions to finish installing the software update, which should take around half an hour depending on the spec of your Mac.