There comes a time when we all have to reinstall the macOS. This usually comes after several tech support procedures that fail. I keep several versions of the macOS on file for this type of thing, but most Mac people would just Restart with Command ⌘ – “R” and reinstall the OS. But, check this out. This article explains here is a way to choose which version of the OS to reinstall which is especially nice if you wish to reinstall the original OS to a machine you are selling.