I think you have buyer and seller mixed up in your post ?
A buyer can ask a seller to cancel, but the seller does not have to agree. If they do agree but do not know how ask them to come to this forum and someone will explain how to cancel the order.
If they have a Returns policy on their listing on receipt of the item, you can return it for the reason, change of mind, but you will need to pay for the tracked return postage.
@mandydevlin