OS X El Capitan has just been released into the Mac App Store. For those of you that wish to do a clean installation or has multiple mac to upgrade, it would be easier to prepare a bootable usb drive. You need to do this before upgrading otherwise the installer will be deleted away.
To do this, simply insert a USB drive and launch Terminal to run the below command (Do note to change the bold text to the name of your USB Drive):
sudo /Applications/Install\ OS\ X\ EL\ Capitan.app/Contents/Resources/createinstallmedia –volume /Volumes/<strong>TechCentral</strong> –applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app –nointeraction
Updated: If you happened to lose the installation file, you could simply search for El Capitan in the app store. In order to just download the installation file, you would have to press on the option key while clicking on download.