You can update your card details through CHI interface:
Go to https://chi.uk2.net/
Login into your account:
Go to the account section, this will be in the bottom left of the new screen
Inside the admin section you need to go to the Payments & Billing tab
Inside the Payments & Billing tab you have the option to Add Credit Card (or Debit Cards) or to Add Direct Debit.
Please select Add Credit Card then complete the form below:
If you have a card listed then next to it under the Action area there will be the options to edit it or delete it. The edit option is to change the active status and alias not change the card itself. (Due to the security methods to protect the card details you can not edit it there)