In order to change the password to the users that are assigned to your database you would need to firstly login to your control panel

Once logged in you will see an option for databases. Hover the mouse over that option and select the type of database the user is linked to.

In the page below you will see the different users that you created:

After clicking on the user in question, you will have the ability to change the password and assign the user to different databases you have:

Click on the Save option to complete any changes you made.