If you have booked your ticket through the official website of KLM airlines, you can easily cancel your flight to the USA / within 24 hours of booking your ticket. This applies to all ticket types under KLM flights.
KLM cancellation policy says the event that your flight is canceled and you decide not to choose an alternative flight
If KLM has rescheduled your flight, it will not suit you.
If you hold a fully flexible ticket or a fully refundable ticket, you can opt for a full refund according to the ticket conditions.