Shut down your Mac, then start it up holding one of these keys:
- option - choose startup disk
- ⌘-r - start macOS Recovery using the latest already installed macOS
- option-⌘-r - start macOS Recovery using the latest compatible macOS
- shift-option-⌘-r - start macOS Recovery using the original macOS
- shift - start Safe Mode
- d - run Apple Diagnostics