By pledging to a campaign, you are committing to supporting that project; cancelling that commitment is discouraged.
However if you have pledged the wrong amount, or pledged twice by accident then you have options to cancel depending on which payment provider you went through:
GoCardless – go to your bank account and cancel the pre-approved direct debit
PayPal – navigate to your pre-authorised payments and cancel your pledge.
This will then be updated on the campaign’s Spacehive page under the ‘backers’ tab.