Restart your Mac and, as soon as you hear the startup chime, press and hold the “OPTION” (or “Alt”) key at boot until the boot menu appears.
Then, select the “Recovery" volume.
(Alternative: If the above doesn’t work… Restart, hold COMMAND + R keys until you see the Apple logo and progress indicator. If neither of those work, try COMMAND + R + OPT.)
Wait for “macOS Utilities” menu to appear (DO NOT CHOOSE TO INSTALL YET)
Select “Disk Utility,” and then click Continue.
Select your startup disk from the list on the left (usually Macintosh HD), and then click the Erase tab.
From the Format pop-up menu, select Mac OS Extended (Journaled), type a name for your disk (usually Macintosh HD), and then click Erase.
After the disk has been erased, choose Disk Utility > Quit Disk Utility in the menu bar at the top of the display.
If you’re not connected to the Internet, choose a network from the Wi-Fi menu in the right corner of the menu bar at the top of the screen.
Select Reinstall Mac OS X, click Continue, and then follow the instructions.
Once the re-install is complete, you will be asked to create a new user.
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