You can only request these refunds in certain circumstances. Check the Advance seat booking policy for more information.

If you booked through a travel agency, you must process refund requests through them. You cannot do this at the airport or on board the plane.
The deadline for submitting a refund request is 14 days from the date of the last flight in your itinerary and all refunds will be made to the same payment method you used to book your paid seating.