If it's still within 30 days of the delivery date start a return on here:
https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041
select "Item doesn't match description" and if necessary escalate it to eBay after 3 complete working days.

If it's more than 30 days it's too late to do anything via ebay but if it's still within 120 days of payment contact your payment provider and ask about a refund or chargeback.