Go to your cPanel -> MySQL Databases section. Scroll down the page till you notice the small phpMyAdmin link. Click on it.

Once in the phpMyAdmin select the PHPAuction 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 `cpg_users` SET `user_password` = MD5( 'new_password_here' ) WHERE `cpg_users`.`user_name` = "admin_username";

"new_password_here" - replace this with the new password you wish to use.
"admin_username" - replace this with the username the password should be updated for.

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 Coppermine with the new password.