Yes, if you decide that you want to cancel or revise your pledge you can do so as long as the campaign is still live. Once a campaign ends, it is no longer possible to make any changes or cancellations.
Just log in or create an account if you pledged as a guest with the email address you used when you made your initial pledge.
Once you've logged in or created your account, go to Manage Account from your profile menu. From there you can view your pledge history and manage your pending pledge(s) under 'Your pledges'.