What happens if you have an old version of installer.app on your system and want the latest version?
The button under the Mojave Circle says OPEN instead of download ???
You now see Mojave is there in the Mac App Store, but instead of Download it says Open. Let’s find out what that means.
After clicking Open I am presented with this message above. As you can see the App Store first searched my system and found that I already have macOS Mojave installer.app. Notice that it searches all locations, not just the Applications folder where the installer app normally is stored. It found the macOS Mojave Installer.app in a folder called test.
Great, we are ready to go right? Not really because I have no clue what version this is. Looking at the creation date gives us a pretty good clue. MacOS Mojave was released on September 24th 2018. This Mojave installer download was created 23 days after release so it’s most likely 10.14.0.