1. Log into the Customer Control Panel with your main account's username and password.
2. Click on the "Account" tab.
3. Click on the "Admins" link.
4. You'll see your username, to the right of it you'll see the "Change Password" link, click it.
5. Type in your current password.
6. Type in your new password.
7. Retype in your new password again.
8. Click Finish.
You've completed updating your main account's username's password.