If you would like to update your billing information, you can access your account right from your software! Head to the Help menu and select "Manage Payment Preferences".
You will be taken to the billing site (if you haven't yet set up your billing account) you will need to do that via email first. Once you have access to your billing account, you can choose "Update Payment Information" in the bottom section of the page.
If you need further assistance with this, you can submit a ticket here at http://www.fundysoftware.com/support/