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 not including taxes . 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 and change fees are non-refundable.  

ModiFly can be redeemed through Manage My Bookings using your reservation code and last name up to 24 hours prior to departure.