I currently run MacOS 10.12.4 Sierra on a mid-2011 iMac. I want to upgrade to 10.13 High Sierra. On the App Store, the only option (after Sept 24) is 10.14 Mojave. Where should I go to get High Sierra? If I go to https://support.apple.com/en_AU/downloads/macos, there are many different download options, but they all are updates, such as 10.13.6.