Delta SkyMiles Award Passes may be revoked without payment for travel within the US at any time prior to departure.
Cancellations must be made for award tickets for travel outside the US at least 722 hours prior to flight departure or return. Award ticket cancellation fees are a flat $150 fee regardless of fare class or destination, but if you are a SkyMiles Platinum or Diamond Medallion member, the fee is waived.
Any miles you used to book the flight will be credited back to your account after you cancel your reservation and pay the $150 cancellation fee.
Delta Air Lines offers the ability to cancel flights online without hassle. Passengers can also request a refund online by filling out the Refund Request Form. Delta airlines cancellation policy
says that the Refunds will only be provided for eligible tickets canceled within the valid period. No refunds will be issued after expiration. Refund requests may take some time to process and will be approved within 7-10 business days. If a Delta flight is canceled due to an unavoidable reason on the part of the airline, Delta can easily cancel the flight and receive a refund with Delta.