OS X Mountain Lion: Reinstall OS X
You can use the built-in recovery disk to install a new copy of OS X while keeping your files and user settings intact.
Important: You need to be connected to the Internet to reinstall OS X.
- Choose Apple menu > Restart, and then hold down the Command (⌘) and R keys while your Mac restarts.
- If you’re not connected to the Internet, choose a network from the Wi-Fi menu (in the top-right corner of the screen).
- Select Reinstall OS X, and then click Continue.
- Follow the onscreen instructions. In the pane where you select a disk, select your current OS X disk (in most cases, it is the only one available).
- To start the installation, click Install.
You can also create an external recovery disk. For more information, see this Apple Support article: