When you need to reinstall macOS on your Mac, you can install it from Apple using the Internet or you can install it from the recovery partition included on the hard drive or flash storage of every Mac. Both ways of reinstalling keep your files and user settings intact.
Important: You must be connected to the Internet to reinstall macOS.
On your Mac, choose Apple menu > Restart.
Immediately as your Mac restarts, do one of the following:
Install the latest version of macOS from the Internet: Press and hold Option-Command-R until a spinning globe appears, then release the keys.
This option installs the latest version of macOS that is compatible with your computer.
Reinstall your computer’s original version of macOS from the Internet: Press and hold Shift-Option-Command-R until a spinning globe appears, then release the keys.
This option reinstalls the most recent version of macOS that came with your computer, including any available updates to that version.
Reinstall macOS from the built-in recovery disk on your computer: Press and hold Command-R until the Utilities window appears.
This option reinstalls the version of macOS stored on your computer’s built-in recovery disk, including any updates that you installed.
Select Reinstall macOS, then click Continue.
Follow the onscreen instructions. In the pane where you select a disk, select your current macOS disk (in most cases, it’s the only one available).