Ensure that you are logged in as an 'Company Admin' user.
Within the top navigation click on ADMIN.
From the left hand menu select Billing Details.
Look for the field called Payment Details and click on the Change button.
This will load four fields you will be required to complete which include:
Card Number
Expiry Month
Expiry Year
CVC (Last three digits on the back of your card)
When you are happy click Update.