When you enter in an offline pledge entry, or when donors create pledges themselves, those pledges show up in the Pledge List. To access the pledge list, go to Reports > Pledges.
You can edit the individual pledges by clicking 'edit' to the right of the pledge in question. Enter in the correct amount, and click 'save' to save your changes or 'cancel' to cancel the changes. You can also delete the pledge entirely. This will delete the pledge both from the pledge list and from the donor account.