A pledge payment can be deleted from the Gift Grid in the Fundraising Module or in the Gifts Tab in the constituent's record. Select that specific pledge payment and click on the trash can icon, typically located on the left column of the gird but may also be the last column depending on how you have your personal view set up. You will get a confirmation message asking you to confirm that you really want to delete the pledge payment. You can cancel or continue and the pledge payment will be deleted. The pledge payment schedule will be updated to reflect the new amount due.