The administrator username of Joomla applications can be easily changed with a simple MySQL query. The easiest way to do this is to use phpMyAdmin or similar MySQL Database modification tool.
Once you access your Joomla database, open the SQL tab where you can execute MySQL queries. In this field type the following query:
UPDATE `jos_users` SET `password` = MD5( ‘new_password’ ) WHERE `jos_users`.`username` = “admin” ;
Replace “new_password” with the new password you would like to use
Replace “admin” with your administrative username
Once you do those changes, execute the query. Now all you have to do is to login to your Joomla site at yourdomain.com/administrator.
Note: These instructions are valid both for Joomla 1.5 and Joomla 1.0.*