You can update (or cancel) your pledge including your payment details as long as the campaign you pledged to has not yet ended. First Log in then navigating to 'Manage Account' and then 'Your Pledges'. Cards are only charged when a campaign is completed and the goal is over 80% funded.
If you pledged using a guest account, you must create account using the email address you used when you made your initial pledge. Once you've created the account, log in and follow the instructions above.