Previously with the Joomla! CMS, it was necessary to go through the process of downloading the latest version and manually going through an upgreade process by loading all the new files to your site.
Fortunately, Joomla! often brings out new versions which have tweaks, but more importantly security fixes where any are identified that need doing.
The old process of updating is no longer required with a brillian extension called Joomla Updater. It is installed as a component on your site and when accessed tells you which version of Joomla you are running and what the latest version is. If you are not up to date, you simply click the update button and your site is automagically updated to the latest version.
This extension saves much time for web developers and webmasters and even allows clients to easily update their own sites instead of paying for it to be done.
The extension is available at http://www.realtyna.com/
I have installed it on all my clients sites and my own sites and found it to work perfectly. I thoroughly recommend this component/extension for Joomla.