Mountain Lion (OS X 10.8) is available in the Mac App Store for only 20 dollars. Unfortunately, the installation of Mountain Lion requires that you already have a Mac running Snow Leopard (OS X 10.6) or Lion (OS X 10.7). If you are currently running a Leopard system, you are unlucky and need to pay $ 29 to upgrade to Snow Leopard, and then an additional $ 20 to upgrade to Mountain Lion again.
That sounds like a big problem to me. But why exactly is Leopard incompatible? It turns out that it is not, in its majority, anyway. The only real compatibility issue is that Mountain Lion is only available in the Mac App Store, which is available from Snow Leopard. However, if you have an alternative machine to make the purchase, or you have put Mountain Lion in a flash drive or DVD, there is really no technical reason why Mountain Lion will not work.
One way to make this work is to perform a clean installation of your Mac, first making a backup of all your files and then transferring them once you have installed the new operating system. However, Mac World invented a much faster and easier way to "cheat" your Mac so he thinks he already has Snow Leopard installed.
However, it involves editing a file.plist, so if you are a little apprehensive when entering inside your operating system, this may not be the right method for you.
Browse to the folder / System / Library / CoreServices .
Step 2: Edit SystemVersion.Plist
Using a text editor that allows you to edit files at the system level, open SystemVersion.plist .
Locate the line with the password Product Version . Just below is a string of characters that indicates which operating system you are running. Your number may resemble 10.5.8, which means that you are running OS X 10.5.8.
Change that number to 10.6.8 and save the file.
Step 3: Restart your Mac
Finally, turn off your Mac and restart it. I will have to boot from a drive that runs Snow Leopard or later, like the Mountain Lion boot drive that I mentioned earlier.
When the OS X Utilities screen appears, select Reinstall OS X to install Mountain Lion in your Leopard unit.