You can simply cancel your order by visiting the order tab after you sign in to your account. Order can be cancelled within 12 hrs of placing it so we can hold your order from further processing. The entire amount will be refunded via payment option selected by you if cancelled before 12 hrs. Once your order has been shipped it can not be cancelled.
For COD, order once reached to the destination, the customer can not refuse to accept the order without a valid reason i.e. different/damaged product.