Before updating to mountain lion
As tempting as it may be to get on with the installation, I recommend quitting the installer at this point and taking care of some setup details first.
El Capitan was announced at WWDC 2015 and will go through a public beta process beginning in July 2015, finishing with a public release on September 30, 2015.
When your Mac first starts up after a clean install, the initial setup wizard will walk you through the process of creating a new administrator account. The clean install option is a very effective way of starting over and may be a good method of installing a new OS if you've been having problems with your Mac that you just can’t figure out.
You can find out more at: How to Perform a Clean Install of OS X El Capitan on Your Mac The third step in upgrading to OS X El Capitan is to check your startup drive for errors and repair file permissions. I’m assuming you already performed a backup and checked to make sure that your Mac meets the minimum system requirements.
OS X El Capitan can be found in the Mac App Store as a free upgrade for anyone who is running OS X Snow Leopard or later.
Should you have a Mac that meets the minimum system requirements for El Capitan, but is running a system earlier than OS X Snow Leopard, you will first need to purchase OS X Snow Leopard (available from the Apple store), and then follow these instructions to install Snow Leopard on your Mac.
But first, you need to take a few preliminary steps to ensure that your Mac is ready to successfully install the OS and that you'll have a trouble-free installation process.
I know, backups are boring, and you would much rather just get on with the installation so you can investigate all of the new features of OS X El Capitan.
Upgrade Install overwrites your current version of OS X, replaces any outdated system files, installs new system files, resets file permissions, updates Apple-supplied apps, and installs new Apple apps.
This is true even if your previous version of OS X was set to bring you directly the Desktop.
Don’t worry; later you can use the System Preferences pane to set the user login environment to the way you want it. Besides the default desktop picture being a spectacular winter view of Yosemite Valley, complete with El Capitan towering in the foreground, the OS itself deserves a closer look. You may find some things aren't working quite the way you remember.
Snow Leopard is the oldest version of OS X that can access the Mac App Store.
Create a Bootable OS X El Capitan Installer on a USB Flash Drive This step is optional but can be helpful if you have multiple Macs to update because you can use the bootable USB flash drive to run the installer from, instead of downloading the OS from the Mac App Store on each and every Mac you intend to update.
But believe me when I say the new OS will wait for you and making sure your current data is safely backed up isn't something to overlook.