Speak with the BT Mobile loyalty team, explain you want to return the handset, you have 14 days from receiving it to change your mind.
When you do return it in the bag BT send, make sure the postage you opt for is tracked and insured, you're responsible for it until BT receive it.
Once BT receive it you can order another, it might be worth visiting a EE/BT store so you can see the device and make sure it's right for you, ask questions and come up with your own list of requirements, do you want a good camera for photos and video calling ? Is battery life important ? etc
