How to reinstall OS X – Apple Support

July 17, 2016

QT:{{”

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.
“}}

https://support.apple.com/en-us/HT204904