how to change operating system from 10.7.5 to10.8

Back up your data,check your applications for compatibility, and try downloading El Capitan from the Mac App Store; if your computer isn’t compatible with it, it can’t run 10.8 either.

If you specifically need Mountain Lion for some reason, buy a download code for it from the online Apple Store.

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First check system requirements for v10.8 Mountain Lion > System requirements for OS X Mountain Lion

If your Mac meet the requirements, you can purchase Mountain Lion v10.8 from the Apple Store > http://www.apple.com/shop/product/D6377Z/A/os-x-mountain-lion

Apple will send you an email a redemption code you can use at the App Store to download Mountain Lion.