macOS Sierra: Reinstall macOS
Use the built-in recovery disk to reinstall macOS while keeping your files and user settings intact.
Important: You must be connected to the Internet to reinstall macOS.
In the menu bar, choose Apple menu > Restart. As your Mac restarts, hold down the Command and R keys until the Utilities window appears.
Select Reinstall macOS, then click Continue.
Follow the onscreen instructions. In the pane where you select a disk, select your current macOS disk (in most cases, it’s the only one available).
Last Modified: Sep 26, 2016