Updating your Worldpay billing information is a bit of a pain because you have to do it on their website. Here are the instructions from Worldpay's site on how to update your subscription:
1 - Log in to the Shopper Management System (http://www.worldpay.com/shopper/index.php?page=recurring&sub=login&c=UK).
2 - The FuturePay Shopper Home Page is displayed.
3 - Under the Agreement ID column, select the hypertext displaying the Agreement ID you want to change the payment details for.
4 - The FuturePay Agreement Details screen is displayed.
5 - Select the Change Payment Details button at the top of the screen.
6 - The Change Payment Details page is displayed.
7 - Enter the new card or bank account details. For more information on the fields you must fill out, please refer to Change Payment Details Page Description. An asterisk (*) after a detail means you must supply that detail.
8 - Select the Submit button to enter your new payment details into the system.
9 - WorldPay will attempt to take the next payment, when due, from the updated card or bank account. If there are any outstanding payments due, we will attempt to debit payment within 24 hours. Once further payment will be taken each consecutive day for any outstanding payment (for each scheduled payment that has been missed).
Hopefully that worked for you. If not, you should contact Worldpay support and get them to help.