Make sure you are signed in the same eBay account you used to make the purchase, find the item in your Purchase History and use the "Return this item" link.
https://www.ebay.com/help/buying/returns-items-not-received-refunds-buyers/return-item-refund?id=404...
If you purchased as a guest, sign out of your current eBay account and see here:
https://www.ebay.com/help/buying/returns-refunds/returning-item/returning-items-bought-guest?id=4065
