Joomla's admin username can be easily changed with a simple MySQL query. The most convenient way to manage the database is through the phpMyAdmin tool. Go to your cPaneland click on phpMyAdmin in the Databases box. (If you are not using cPanel or do not have phpMyAdmin, you can run the query directly for Joomla's database.)
Once in the phpMyAdmin select the Joomla database from the drop-down menu at left. The page will refresh and the database's tables will be displayed on it. Open the SQL tab (look at the top navigation bar).
In the text field write the following SQL query:
UPDATE `jos_users` SET `password` = MD5( 'new_password' ) WHERE `jos_users`.`username` = "admin" ;
"new_password" - replace this with the new password you wish to use.
"admin" - replace this if your admin username is different.
Once you are ready, click on the GO button to submit the query. If everything goes fine without errors, you should be able to login to Joomla with the new password.
Note: These instructions are valid both for Joomla 1.5 and Joomla 1.0.*.
These instructions were taken from the SiteGround Knowledge Base.