You have two choices: 1. through your account or 2. by using WorldPay:
Through your account
Log in to your account on moonfruit.com
Go to Billing (within the Account section)
Under 'Plan', click the option: Renew with your new card details
Follow the instructions and provide your new card details. This will set up a new FuturePay Agreement (repeat billing) and cancel the older version.
Make sure the confirmation email you receive from WorldPay is kept safe should you need to access your WorldPay Shopper account in the future
When you originally set up your payment or if you have updated your card details in the past, you will have received a confirmation email from: email@example.com. The email would have had the subject: 'WorldPay Regular FuturePay'. It will have contained your WorldPay Shopper login details.
If you need to update your credit or debit card details (e.g. you've been issued with a new card) you can visit the WorldPay Shopper Login page and log in using your WorldPay username and password supplied in the email described above. This is different from your Moonfruit SiteMaker login!
This will allow you to locate your FuturePay 'Agreement ID' which you can click to access the details, and check the Merchant Name (SiteMaker Software Ltd), your credit/debit card number (last 4 digits) and the card expiry date, amongst other details.
Clicking on the 'Change Payment Details' button at top of page will take you to that page where you can update card details e.g., the card number or the expiry date. Once submitted these details will be used for all future transactions on that FuturePay agreement until changed or cancelled.
NB. If you change your card details on an agreement over 24 hours after the expiry of your account then this process may not work. You are better updating your card details through Moonfruit to set up a new Agreement.