2. Decide how to upgrade
You can upgrade to your new operating system by clicking Apple > Mac App Store and typing ‘High Sierra’ into the search bar. Click Get below the High Sierra icon. You’ll need around 5GB on your hard drive (see step 6 for tips on freeing up space). Follow the steps in the installation wizard to perform your upgrade.
If you have already installed the experimental public Beta version of High Sierra, you’ll need to prepare a bootable USB version of the operating system and perform a fresh install (see the next step). You can also do this at a later stage if you have already installed High Sierra but the system is performing poorly.