It is risky to upgrade a live site without testing the upgrade process first. So before upgrading the live site you need to set up a test site which is as close as possible to the configuration of your live site, then test the upgrade on that. When you are happy with the process you can apply it to the live site (having taken a fresh backup of the live site first, of course).
|Information: What is a patch?|
Determine the current version of your Joomla! installation. For example, if you currently have version 1.5 RC4 installed you will need to download the 1.5 RC4 to 1.5 Stable patch package. The patch packages are available on <insert link to JoomlaCode here>. Each patch package is available in three different package formats. These are indicated by the file extension which is either .zip, .tar.gz or .tar.bz2. The contents of each of these alternative packages is identical so just choose the one most convenient for you.
|Information: More about package file formats|
For example, if you are upgrading from 1.5 RC4 to 1.5 Stable and you would prefer the .tar.gz package format, then select the file named Joomla_1.5RC4_to_1.5Stable-Patch_Package.tar.gz <Check the filename>.