You can update (or cancel) your pending pledge as long as the campaign you pledged to hasn't ended.
To edit a pending pledge, you'll first want to login to the Seed&Spark account associated with the email address you used to pledge. Once logged in, you can click here to review your pledges.
Then you would just need to click the "Manage" button that looks like this:
If you pledged as a guest, you must first create a full Seed&Spark account using the same email address you used when you placed your pledge. Once you've created the account, logging in and following the above instructions will allow you to edit your pending pledge.
If for any reason your credit card is declined after a campaign ends, you'll still have the ability to reenter your payment details to fix it.