In the Joomla! admin panel:
1. Login and click Site and then User Manager.
2. Check the box next to Administrator (under Name).
3. Scroll right and click on Edit (next to Logout and Delete, upper right).
4. Enter a new username and/or password in their respective fields in the User Details section (you will also need to enter your new password in the Verify Password field.)
5. Click on Save in the right corner.
Your admin username in Joomla! can also be changed with a MySQL query through the phpMyAdmin tool. (If you do not have phpMyAdmin, you can run the query directly for your Joomla's database.)
1. Once in phpMyAdmin, select your Joomla database from the drop-down menu on the left. The page will refresh and the database's tables will be displayed on it.
2. Click on the SQL tab (look at the top navigation bar).
3. In the text field, write the following SQL query:
UPDATE `jos_users` SET `password` = MD5( 'new_password' ) WHERE `jos_users`.`username` = "admin" ;
Change "new_password" to the new password you wish to use.
Change "admin" to the new admin username if you want to change this as well.
4. Once you are ready, click on the GO button to submit the query.
If there are no errors, you should now be able to login to Joomla! with your new password.
Note: The instructions for the SQL query are valid both for Joomla 1.5 and Joomla 1.0.