Preparing for Installation
After you have confirmed that your Mac meets the system requirements for OS X Mountain Lion, you need to prepare your computer for the update.
The first step is to ensure that your computer's firmware is up to date. Firmware is a computer program that resides in the Mac's motherboard and controls it; as with application software, it can be updated with new instructions.
The easiest way to ensure that software and firmware are up to date is to use the built-in software update feature. In OS X Snow Leopard or Lion, choose 
⇒Software Update. If any updates are available, they appear in the main window; click the Install button for the update, and, if requested, enter your administrator password. You also can download major updates from www.apple.com/support/downloads.
Caution
Do not interrupt a firmware upgrade; you could permanently damage your Mac by doing so. If you are installing a firmware update on a MacBook, make sure you connect it to a main power source so there's no chance of the battery dying mid-upgrade.
Setting up a Mac for installation
It is likely that you are installing OS X Mountain Lion on a hard disk (your startup disk) already containing a previous version of OS X, replacing the existing OS X with OS X Mountain Lion. Doing so keeps all your settings and doesn't require reinstalling all your applications.
Note
You can install OS X on ...
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