There’s no need to visit a store, bring home a box, and install a bunch of discs. Just click the Mac App Store icon on your Mac. Buy OS X Mountain Lion. And your Mac does the rest. Follow these steps to begin your upgrade.
How to upgrade to OS X Mountain Lion
Make sure your Mac can run Mountain Lion.
Your Mac must be one of the following models:
iMac (Mid 2007 or newer)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)
If you are running Lion, you can find out if your current Mac qualifies by clicking the Apple icon at the top left of your screen, choosing About This Mac, then clicking More Info.
Make sure you have Lion or the latest version of Snow Leopard.
While you have the About This Mac window open, check what version of OS X your Mac is running.
If you are running Lion (10.7.x), you’re ready to update to Mountain Lion. Go to Step 3. If you are running Snow Leopard (10.6.x), update to the latest version of OS X Snow Leopard before you purchase OS X Mountain Lion from the Mac App Store. Click the Apple icon and choose Software Update to install Snow Leopard v10.6.8, the latest version.
Download OS X Mountain Lion from the Mac App Store.
Open the Mac App Store from your Dock to buy and download Mountain Lion. Then follow the onscreen instructions to install it. If you don’t have broadband access, you can visit any Apple Retail Store to get help with downloading.