To initiate a valid return, reach out to the brand owner by replying directly to your receipt email or by clicking on the "Ask a question" or "Contact shop" button on any product page. Read more about how to contact a shop here.
You can find a shop's Terms & Conditions and Return Policy in the bottom left-hand corner of its brand page. You can also find return policies on individual product pages, under the Shipping and Policies section.
Once an order has been placed, you can also find the shop's Return Policy in your email receipt.