The Joomla! development team regularly releases updates to fix bugs and security issues. At the time of writing, the current version number is 1.5.15. This indicates it is the 15th update of the original 1.5.0 release. Try to keep up with these updates; you'll find announcements of new updates on the Joomla! site (www.joomla.org). The essential steps of upgrading are downloading the upgrade files (contained in a zip file), unzipping these, and uploading the updated files to the web server—overwriting the existing files.
Don’t forget there's more to keeping your site current than just upgrading the Joomla! software itself. It's a good idea to check for updates for the extensions you use in the Joomla! Extension Database. To read more on upgrading, visit the Joomla! documentation site:
http://docs.joomla.org/Upgrading_1.5_from_an_existing_1.5x_version.