If you have access to mysql through the root user and want to just change the drupal database user this will work for you.
Otherwise check out [http://www.thegeekstuff.com/2009/07/how-to-reset-forgot-mysql-root-password-on-unix-linux-windows/](http://www.thegeekstuff.com/2009/07/how-to-reset-forgot-mysql-root-password-on-unix-linux-windows/)
open phpmyadmin or login to mysql using the root user.
select the mysql database and use the following cmd
UPDATE user SET password = PASSWORD('newpassword') WHERE user = 'drupal_db_username'
update your drupal mysql database password in drupal by finding the file
Specifically, look for the line:
$db_url = 'mysql://username:password@localhost/databasename';
update it with a text editor then restart mysql and your web server