Step 1: The first step whenever you install a new OS version is to take a back up of your data, so you can do that now via Time Machine on an external drive. This is the official way to upgrade macOS from old version to a new version. Since the first release of macOS, Mac users can directly upgrade to a new version via macOS installer app, which you can get from App Store. Method 1: Direct Upgrade from macOS High Sierra to macOS Mojave