Patching is a normal and regular part of running any software. Your website is made up of many software programs working in concert to create and display your online presence.
Patching actually means to update your software to the latest version. In this recipe, we'll look at how to patch your core Joomla! files and your extensions.
As Joomla! has regular updates, it's important to monitor and apply patches to the core Joomla! files.
Download the patches or full files from joomla.org. As an example, if you are running 1.5.17 there usually is a patch that will take you from 1.5.17 to say 1.5.18 (or whatever the latest is in the 1.5 series). You can download the entire version 1.5.xx (latest at the time you download it) and upload it to your site.
Make a full backup of your database and website and pull the copy off to another computer.
Unzip the files.
Connect with FileZilla FTP client to your website.
Copy all the files and overwrite your current files.
Test the site once...