That can be frustrating, but it’s the price of progress.
Remember that you’ll need at least 12GB of storage space available to install Mountain Lion, but realistically you should have more than that available to insure your Mac runs best anyway.
Learn more about how to upgrade to mac OS Sierra If you’re running Snow Leopard (10.6.8) and your Mac supports mac OS Sierra, you will need to upgrade to El Capitan first. If you’re running Leopard and would like to upgrade to mac OS Sierra, first you’ll need to upgrade to OS X Snow Leopard.
You can purchase OS X Snow Leopard from the Apple Online Store.
If you haven't yet upgraded to OS X Lion, the new distribution and installation methods may seem a bit foreign.
On the plus side, Apple worked out most of the glitches on Lion, so you get the benefit of installing Mountain Lion using a well-understood and reliable method.
The upgrade install process lets you install Mountain Lion over your existing version of OS X, and still retain all of your user data, most of your system preferences, and most of your applications.
You may lose some of your apps if they can't run under Mountain Lion.
Either way, this step-by-step guide will help ensure that you understand how everything works.
There are several ways to install OS X Mountain Lion.
This guide will show you how to perform an upgrade install, which is the default installation and the one that Apple thinks most Mac users will choose. You can also perform a clean install, or install the OS from various types of media, such as a USB flash drive, a DVD, or an external hard drive. OS X Mountain Lion is the second version of OS X that can only be purchased through the Mac App Store.
The installer may also change some of your preference files because certain settings are no longer supported or are incompatible with some feature of the new OS.
Most of you won't have any problems with installing and using Mountain Lion, but there's a small chance that your particular combination of apps, data, and preferences will be the one that was never thoroughly tested before Mountain Lion was released.
A good list of compatible and incompatible apps is maintained in a searchable database by Roaring Apps, check it out here.