I've added the details of my new Guaranty Trust Bank Mastercard to my billing account and everything is now OK. So, in this post, I decided to share with you, the steps in adding your debit or credit card details to your Google Apps for business account.
To update your form of payment:
==> Access your Admin Console at admin.google.com.
==> Select Billing.
==> Select the $ icon to access your billing account. This will take you to your Transaction history tab.
==> Select the Billing settings tab.
==> To add your credit or debit card, click Add new form of payment
==> Enter your card details etc and select the box "Make this my selection for future payments".