How do I get a refund or sell my iPhone in the Apple store?

How do I sell or get a refund on my iPhone? I want to refund or sell it to the Apple store to get the exact same phone in different color. Does it have to be in the same store where I got my phone or could it be a different Apple store? Also once I refund my iPhone I get all the money back? Or once it happens and get they same phone in different color I have it put a amount of money into it so it could complete the amount?