There are times that Frontier will have to cancel or make changes to a flight. These changes or cancellations can happen for many reasons and are handled with care because we realize there may be impacts to our customers.
In the event, your flight is canceled, or your domestic flight is delayed more than 3 hours (180 minutes) or your international flight is delayed more than 6 hours (360 minutes), you are eligible for a refund, to rebook, or to cancel and receive a credit shell valid for 5 years from date of cancellation
Simply follow the link when notified.