Open the app and go to the Profile section on the menu. At the top you will see buttons showing how many items you are offering, you have given away, or you have Called Dibs on. By default the initial view shows all items you hare offering. If it is not showing, simply tap the button to get to that view.
Items with the "Dibs!" icon overlaying the image indicate that someone has Called Dibs on that item. If you want to give the item or see who has Called Dibs, view the item detail to get the "Give It" button to view who has called dibs and, optionally, select a recipient.
If you simply want to cancel the giving, tap the wrench icon in the upper right to delete the listing and cancel the offering.