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OS X Mountain Lion and Lion are paid upgrades, so you can't get them from Software Update.
First, if you are happy with Snow Leopard, there is no reason to upgrade, but if Mountain Lion has a feature that you need, upgrade. Open App Store and purchase Mountain Lion. If your Mac isn't compatible, the App Store will show a message and Mountain Lion won't be purchased.
While OS X is downloading, you can make a backup of your files. When the OS X installer has downloaded, the installer will show, so follow the steps. Check if your programmes are supported > http://www.roaringapps.com