Yes, you may cancel your order if you act quickly, before it has been processed and shipped out by the seller. To do so, head to your orders page and click "Need Help?" next to the item you wish to cancel. Then submit the request "I have another comment or question about this item" and let us know you wish to cancel the order.
More information about requesting help on your order can be found here.
If your item has already been sent out, then you will have to request a refund after receiving the item.