Here are the steps for a customer to update their billing information in the WHMCS customer portal:

1. Log In to the Customer Portal:

   Customers should access the Client Portal by visiting https://www.durnwood.com/portal/

2. Log In to the your Account:

   You need to log in to your account using your email address and password. If you have forgotten your password, there is an option to reset it via email.

3. Access Billing Information:

   After logging in, you should look for a "Billing Information" or "Account Information" option in the client portal's navigation menu. 

4. Edit Billing Information:

   In the "Billing Information" or "Account Information" section, you can usually edit your billing information, which will include details such as their billing address, contact information, and payment methods.

5. Save Changes:

   After making the necessary updates to their billing information, you should look for a "Save" or "Update" button to save your changes.

6. Verify Changes:

   Customers should review the updated billing information to ensure that it is accurate and up to date.

7. Optional: Update Payment Methods:

   If you need to update your payment methods (e.g., credit card information), you should navigate to the "Payment Methods" or "Payment Information" section in the portal, where you can add, update, or remove payment methods as needed.

If you have trouble updating your billing information, you may want to contact customer support for assistance.