Upgrade Using Plan B: Clean Install
If you prefer a cleaner installation of Mojave than what an in-place upgrade provides—specifically, one that gets rid of any disk errors and gives you a chance to weed out some types of user-created data—Plan B is what you should follow instead of Plan A. It’s also the right choice if you tried Plan A and encountered problems.
Much of Plan B is just like Plan A, so I refer to earlier discussions for some details. The differences come at the beginning (erasing your disk manually) and the end (using Setup Assistant to configure numerous settings and, optionally, transfer data from your bootable duplicate).
Start Up from Your Bootable Duplicate
Unlike Plan A’s in-place upgrade, Plan B requires a bootable duplicate ...