For sneakers, you may cancel your order within 3 hours of placing it or before it is confirmed by the seller - whichever comes first. If 3 hours have passed from when you placed the order and/or the seller has confirmed the order, then the order cannot be canceled. If neither have occurred, simply tap the Cancel button on the Orders screen. You may not cancel an offer after it has been accepted by a seller.
For apparel and accessories, all orders are auto-confirmed and cannot be canceled.