MacOS - Recovery Mode
- Power down, hold one of the following:
- Command-R: gets you into Recovery Mode and various options like Disk Utility and re-installing your current version
- More from Apple Support: Reinstall MacOS
- Option-Command-R: “Install the latest version of macOS compatible with your computer”
- Option-Shift-Command-R: “Reinstall your computer’s original version of macOS (including available updates)”