You can change your seat once you have booked and/or selected it, up until 48 hours before your departure time.
You can login to your booking through the 'Manage Booking' section on website.
If you wish to change a seat, this will show a refund for the first seat, then charge again for the new seat. If you choose a seat of higher value, this will charge the higher price.
Please note that seat reservations are 100% non-refundable, should you remove the seat assignment and not select a new seat.