The Joomla Project announces the immediate availability of Joomla 1.6.4. This is a security release. This release corrects four security issues and fixes an issue relating to version upgrades. Note that the upgrade to version 1.6.4 should be done before upgrading to version 1.7.0 (due out in July).
Important Note: A security fix was made for the Category List layouts for articles, contacts, newsfeeds, and weblinks as well as the Featured Contact list. If you have created a template override for one of these layouts based on the core layout, your file could contain this security issue. Please see the release FAQ for more information.
The Production Leadership Team's goal is to continue to provide regular, frequent updates to the Joomla community. Learn more about Joomla! developement at the Developer Site.
Here are upgrade instruction from previous 1.6 version:
Step 1: Make sure you have a current backup of your site
Step 2: Find and install updates
In the back end of your Joomla site, navigate to Extensions > Extension Manager and open the Update tab.
[li]Click on the Purge Cache icon to clear out the cache.[/li]
[li] Click on the Find Updates icon in the toolbar.[/li]
[li] Select the update (using the check-box) and click on the Update icon in the toolbar.[/li]