You can use macOS Recovery, the built-in recovery system on your Mac, to reinstall macOS. macOS Recovery keeps your files and user settings intact when reinstalling.
Important: Your computer must be connected to the internet to reinstall macOS.
Start up your computer in macOS Recovery:
On a Mac with Apple silicon: Choose Apple menu > Shut Down, press and hold the power button until “Loading startup options” appears, select Options, click Continue, then follow the onscreen instructions.
On an Intel-based Mac: Choose Apple menu > Restart, then immediately press and hold one of these key combinations, depending on what you want to do:
Install the latest version of macOS compatible with your computer: Option-Command-R.
Reinstall your computer’s original version of macOS (including available updates): Option-Shift-Command-R.
Reinstall your current version of macOS: Command-R.
In the Recovery app window, select Reinstall for your macOS release, then click Continue.
Follow the onscreen instructions.
In the settings where you select a volume, select your current macOS volume (in most cases, it’s the only one available).
On a Mac with Apple silicon or an Intel-based Mac with the Apple T2 Security Chip, use Erase Assistant to reset your Mac to factory settings before you trade it in or sell it. See Erase your Mac.