You can cancel a pledge by visiting the project page and clicking the "Manage your pledge” button. From there you can click the "Cancel Pledge" link located next to the "Change your pledge" button.
If you pledged to a project as a guest, please click the “Manage your pledge” link next to “Pledge details” in your confirmation email. From this section on the manage pledge page, click “Cancel pledge” next to the “Change your pledge” button.
Please note that a pledge can only be cancelled while the project’s funding period is live.