Substitute new_ placeholder in the second line with your new prefix. Run it: http://www.yoursite.com/rename.php and wait until it responds with an OK, usually after a couple of seconds. Delete the rename.php script. Edit the configuration.php file and find the line starting with var $dbprefix. It should look like this: var $dbprefix = \'jos_\'; Replace the old jos_ prefix with new prefix, i.e. the one used in the second line of your rename.php script.