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Important: You need to be connected to the Internet and to a power source to reinstall OS X.
You cannot reinstall an earlier version of Mac OS X over a later version unless you perform an archive install or reformat the computer.
Read the "Before You Install" or "Read Me" document associated with your operating system if one is available for the software you are installing. This may list the system requirements and any known problems.
If you receive a warning message about updating your computer's firmware, be sure to do so prior to installing the operating system. For the latest Firmware Updates, run Software Update.
Allocate a few hours or more to reinstall Mac OS X, software updates, and your application software.
Back up all important documents to a separate device before beginning.
Have all application licenses available for reinstalling if you plan to do a format and installation of Mac OS X. This involves deleting all files on your hard drive and copying an entirely new set of operating system files onto your computer. This replaces the previous operating system and is preferred in general, as it will most likely eliminate previous system software problems.