To use a flight credit from an unflown flight, simply book your new flight on Flyfrontier.com. Once you reach the payment screen, select "Credit with Frontier" and enter the confirmation code from the original booking (six characters, a combination of letters & numbers) to use the credit.
PLEASE NOTE:
Credits are not redeemable for cash.
Credits may not cover the entire value of the purchase. Any remaining balance due must be paid by the customer with a credit card.
Credits are non-transferable and may only be redeemed in the name of the passenger to whom it was issued.
The name on the new reservation using the credit must exactly match the name on the old reservation.