You need to contact the seller and ask them to cancel.    Go to the "More actions" dropdown menu arrow on the right of the item on your purchases list and either use the request cancellation link or the contact seller link.

Be aware,  you can ask a seller to cancel your order,  but they are not obliged to.

If the item is sent, on receipt if the seller has a Returns Accepted policy on their listing,  you can return the item for a full refund,  but you do have to pay the return tracked postage.

@oluwaden-68