Read the full guide over at OSXDaily : http://osxdaily.com/2018/10/10/how-reinstall-macos-mojave/
This walkthrough will show you the easiest way to reinstall macOS Mojave from a Mac that already has macOS Mojave on it by using Recovery Mode.
Note that reinstalling macOS Mojave with this approach is not the same thing as performing a clean install. A clean install of Mojave erases literally everything on the computer, including personal data, by using a bootable Mojave installer drive, while reinstalling aims to simply reinstall only the macOS operating system itself.
Be sure to backup your Mac first, it’s easy to back up with Time Machine. While reinstalling macOS Mojave system software only aims to only replace the system software portion of a computer, it’s possible that something could go wrong resulting in data loss. Therefore you absolutely Backup the Mac before going any further, do not skip making a full backup