The trade in box you received has a label under the label with your address on it. You put your trade in the box, following the directions provided in the trade in kit and take the box to Fedex and they'll ship your phone to the Trade in Partner. Once evaluated, they'll either confirm the trade value or if there is an issue with the phone, offer you a different value, which you can accept or refuse. And if you refuse, you can ask them to send the phone back to you.
Or if it would be easier, you can take the phone with your to an Apple Store and complete the trade in the store. Make sure if you go to an Apple Store, you take a copy of the new phone order with you.