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.
Important: You must be connected to the Internet to reinstall macOS. Both ways of reinstalling keep your files and user settings intact. 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.
