MySQL Database Wizard
If you want to create a new database and database user, you can do this is by going to the MySQL Database Wizard under the Databases section in cPanel.
That will walk you through the process of creating a database, creating a database user, setting a password, and then giving the database user permissions to the database all in one set of steps.
MySQL Databases
If you need to set a new password for an existing database user, you can find a list of current databases and database users under Databases > MySQL Databases in cPanel.
If you scroll to the bottom of that page, you will find a list of all Current Users and can click the Change Password link there to set a new password for any of them: