Mojave

If you are referring to the actual Mojave installer (setup) file that is downloaded from the App Store it is located in Finder, in the Applications folder. You can drag the Install macOS Mojave app icon from there to the Trash and delete it. You can then re-download it from the App Store.

If on the other hand you already are running Mojave, and your install itself has become corrupted you can do an over-top reinstall by booting up into Recovery Mode (COMMAND-R) during boot up and select "Reinstall macOS". This will apply the Mojave system files and generally should replace any corrupted files from your current Mojave installation without losing any apps or data.