Sorry you have buyer's remorse. Its actually a common thing and not only for cars.
Unfortunately, usually its not possible to return a car. If its possible, it should be spelled out in your sales contract. You can always try calling the dealer, but be prepared for an answer that you don't want to hear. Sometimes I've heard of a car being very quickly swapped for another with more comfy seats, for example, at the same dealer, but it seems to be rare.