Using your favorite editor, open configuration.php.
Joomla 2.5 look for
public $user = 'database user';
public $password = 'database password';
Change 'database user'; and or 'database password'; to new user and or new password
Save the changes
Unable to Connect to the Database: Could Not Connect to MySQL
If during or after installation, you received "Unable to connect to the database" error, verify that you have entered your MySQL database details correctly. The installation script will not allow you to continue unless the details are correct. The error might be due to writing a password which doesn't exist: you are not expected to create a password for the database during installation.
If the failure occurs after moving your site to another host, check the following items of your configuration.php file. The normal database settings are the following:
var $dbtype = 'mysql';
var $host = 'localhost';
var $user = 'user_with_database_access';
var $db = 'the_actual_database_name';
var $dbprefix = 'table_prefix_';
The Most Common Reasons
Sometimes you will see this message if MySQL has stopped running on your server. Your server administrator may temporarily turn MySQL off to run maintenance utilities. In such circumstances, your site will likely return shortly.
Your database user has been deleted. If this is the case, you will need to recreate your database user with the same username and password that existed when you first installed Joomla. Use your domain control panel to administer this or contact your server administrator.
Your database username or password has changed.