If you added ModiFly at the time of your booking, it can be used to make changes. Otherwise, standard change fees apply:
Under our standard change policy, changes being made 3 days (72 hours) or more from departure, and less than 7 days from departure, incur a fee of $150 per passenger per segment. For changes being made 7 days (168 hours) or more from departure, changes incur a fee of $100 per passenger per segment. No changes are permitted less than three days (72 hours) prior to departure.
ModiFly can be redeemed through Manage My Bookings using your reservation code and last name up to 24 hours prior to departure.