How do I find out the user name and password for an existing account?
If you have access to the database you can change the password with a simple query.  If you want to save one step from having to go to the MD5 website you could also use the MySQL built-in function with something like this:

UPDATE `users` SET `pass` = MD5('mynewpassword') WHERE `uid` =1;


Note:  That only seems to work for Drupal sites &lt;= version 6.x.  If you are using version 7.x the MD5 hash will not work (I'm guessing it uses some sort of salt string on it).  To reset the password on version 7.x you can just change the email address of the admin user to your email address with a query like this:

UPDATE users SET mail = 'myemail@mywebsite.com' WHERE uid = 1;


Once that is done you can go to the normal login page and just request a password reset, and it will be sent to your email address.

Reference:  https://superuser.com/questions/104482/how-do-i-take-control-of-a-drupal-installation-on-my-web-space-without-the-admin