How to reinstall OS X – Apple Support
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Start up from OS X Internet Recovery by holding down Option-Command-R immediately after turning on or restarting your Mac. Release the keys when you see the Apple logo. Startup is complete when you see the OS X Utilities window.
Open Disk Utility from the OS X Utilities window, then use Disk Utility to erase your built-in hard disk using the OS X Extended (Journaled) format. Quit Disk Utility when done.
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