Boot into recovery mode

Intel based

Reboot and press:

• Command-R:
  Start up from the built-in macOS Recovery System. Use this key combination to reinstall the latest macOS that was installed on your system, or to use the other apps in macOS Recovery.
• Option-Command-R:
  Start up from macOS Recovery over the internet. Use this key combination to reinstall macOS and upgrade to the latest version of macOS that’s compatible with your Mac.
• Option-Shift-Command-R:
  Start up from macOS Recovery over the internet. Use this key combination to reinstall the version of macOS that came with your Mac or the closest version that’s still available.

Silicon based

1. Turn off your Mac.
2. Press and hold the Power button.
3. A message stating you'll be able to access startup options soon will appear. Keep holding the button down.
4. Click Options > Continue to open up Recovery. 

Useful Link

All startup key codes