Mountain Lion is the second version of macOS that can only be purchased through the Mac App Store. The upgrade install process lets you install Mountain Lion over your existing version of macOS and still retain all of your user data, most of your system preferences, and most of your applications. You may lose some apps that cannot run on Mountain Lion. The installer may also change some of your preference files because certain settings are no longer supported or are incompatible with some features of the new OS.
What You Need to Perform an Upgrade Install of OS X Mountain Lion
A copy of the Mountain Lion installer, which is available from the Mac App Store. You must be running Snow Leopard or later to access the Mac App Store, but you don’t have to install Lion before you install Mountain Lion. Mountain Lion will install correctly as long as you’re running OS X Snow Leopard or later. A destination volume for the installation. The Mountain Lion installer can work with internal drives, SSDs (Solid State Drives), or external drives with USB, FireWire, or Thunderbolt interfaces. Basically, any bootable device will work, but because this is a guide to an upgrade install, the target volume must be running OS X Lion or earlier. If your Mac doesn’t meet this requirement, then the Clean Install guide is a better choice for you. A minimum of 8 GB of free space, but more space is better, of course. A minimum of 650 MB of free space for the Recovery HD volume. This is a hidden volume that is created during the installation. The Recovery HD volume contains utilities to repair drives and to re-install the OS if you have problems with a drive.
If you have everything lined up with current backups in place, you can proceed to the upgrade process.
How to Upgrade Install macOS Mountain Lion
This guide will take you through an upgrade install of macOS Mountain Lion. The upgrade will replace the version of macOS your Mac is currently running, but it will leave your user data and most of your preferences and apps in place. Before you begin the upgrade, make sure you have a current backup of all your data. While the upgrade process shouldn’t cause any problems, it’s always best to be prepared for the worst.
How to Set up macOS Mountain Lion
When Mountain Lion is installed, either the log-in screen or the Desktop will appear, depending on whether you previously had your Mac configured to require log-in. If you did not have an Apple ID set up for your current OS, the first time your Mac starts up with Mountain Lion it will ask you to supply an Apple ID and password.
Update the Mountain Lion Software
Before you get busy with macOS Mountain Lion, you should run the Software Update service. This will check for updates of the OS and many supported products, such as printers, that are connected to your Mac and may need updated software to work correctly with Mountain Lion. You can find Software Update under the Apple menu.
