Login at SuperAccount Control Panel with the main account's username and password.
Click on the "Website" link.
Click on the "Email" link.
Then click on "Manage Mailboxes".
Select domain of the email accounts you want to manage.
You'll see a list of your email addresses for your domain.
Delete the account that you want to change the password to.
Add the email address back with the new password.
You've have now changed your password on the email account.
NOTE: This will NOT delete any email from their inbox or folders.