how to uninstall OS X Mountain Lion

Install or Reinstall Lion/Mountain Lion from Scratch

If possible backup your files to an external drive or second internal drive.

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Erase the hard drive:

1. Select Disk Utility from the main menu and click on the Continue button.
2. After DU loads select your startup volume (usually Macintosh HD) from the left side list. Click on the Erase tab in the DU main window.
3. Set the format type to Mac OS Extended (Journaled.) Click on the Erase button and wait until the process has completed.
4. Quit DU and return to the main menu.

Reinstall Lion: Select Reinstall Lion/Mountain Lion and click on the Install button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.

If Mountain Lion is what you now have then that is what will be reinstalled. In order to install Lion you would need the Lion installer application saved on your computer. If you did not save the Lion installer then there is only on way to get it now:

If you need to purchase Lion call 1-800-692-7753 or 1-800-676-2775. Ask for a Sales Representative. They will check your Apple ID, and email you a code and instructions. The cost is $29.99 (as it was before) plus tax. It's a download.

I believe as long as you purchased Lion or if it came pre-installed, then they will not charge you the $29.99.

If you upgraded from Snow Leopard then you could reinstall Snow Leopard from your DVD.

The process will erase your drive and reinstall Mountain Lion from scratch. No installer DVD is required. Installation is via the Internet.