ModiFly can only be added at time of booking.
If you did not add ModiFly, flight changes are subject to standard change fees plus any difference in fare. For changes being made 3 days (72 hours) or more from departure, and less than 7 days from departure, changes 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.
To make the change, log in to manage my bookings and select flight changes.