I ordered a bike from amazon and when it arrived i realised i had ordered the wrong size,?

it is still in its box and has not been touched. does any one know how i go about returning it and how much it wil cost to return it, the whole package weighs in at 20 kgs. also does anyone know how long it will take to refund the money to my card so i can order another one that is bigger and how do i pay for the return?