Joomla is one of the most secured CMS out there, but of course, you also have to update your website for the patches. Patches can be easily applied within a minute or two. Here's a 3 step guide on how to update your Joomla CMS

1.  Go to Joomla.org website and download the latest patch. You can easily find the download link by clicking on the button  . As of this moment, the latest build is 1.5.14
2. In there, you can see the Download other Joomla 1.5.x packages » Link, just click it and download the current build upgrade, you can see the version of your joomla website on the upper right corner of your admin panel. Example, if you're using 1.5.8, then you'll have to download joomla_1.5.8_to_1.5.14-Stable-Patch_Package.zip (assuming that 1.5.14 is the latest)
3. Unzip and upload every file in the main site using ftp (just overwrite everything), and you're done!