Apple's newest operating system for the Mac, OS X Mavericks, introduced several new features. But it also introduced some bugs and even took away some features without good reason. If you've been frustrated by Mavericks, or have found that an application you frequently use doesn't work with it, here's how to downgrade back to OS X Mountain Lion.
First make sure you have a backup of all of your data. Time Machine is a pretty simple solution for this, but Leo has also recommended Super Duper in the past.
If you have an OS X Mountain Lion restore disc, just insert that and run the installer. When the computer restarts, go into options and choose "Custom Install" and select the erase and install option. Apple is actually still selling copies of Mac OS X Mountain Lion at store.apple.com for $19.99.
Alternatively if you have a Time Machine backup from OS X Mountain Lion, you can use that to restore your system. Just reboot the Mac and hold down Command + R to boot into the restore menu. Then choose the most recent backup with OS 10.8.
Another option is to use Apple's Internet Recovery to reinstall the original version of Mac OS X that came with your computer. To do this, hold down Command, Option, and R while the computer is booting up. Then open Disk Utility, choose your hard drive and erase the drive. Then continue installing OS X.
In general, it's a good idea to wait before upgrading an operating system until the second update has been released. This allows bugs to be fixed and applications to be updated to run as expected on the new operating system.