If you have booked your ticket through KLM Airlines official website, you can easily cancel your flight to/from US within 24 hours of booking the ticket. KLM Airlines Cancellation Policy applies to all types of tickets under KLM flights. If you have a full flexible ticket or a refundable ticket, you can opt for a full refund according to the conditions of your ticket.

https://www.flightzticket.com/klm-airlines-cancellation-policy