The download will begin, and it will alert you once it is completed. Click on Get, and it should turn green and ask you if you want to install the update. Since the upgrade is free, it should only say Get next to the download icon, instead of listing a price. If it isn't, type 'macOS High Sierra' into the search bar at the top right, and it should be the first option available in the search results. Once available, the link to download macOS High Sierra will likely be prominently displayed on the Featured tab in the store. Start by clicking on the Mac App Store icon on the dock on your Mac. The macOS High Sierra download will be available through the Mac App Store.
