KLM’s 24-hour rule allows passengers to cancel or change their flight within 24 hours of booking without fees. For assistance, contact KLM customer service at 1-844-231-5895 (US) or +44 (20) 39003980 (UK) for more information and support.

https://myrbkc.rbkc.gov.uk/forums/support-forum/891c49c1-fbde-ef11-a81b-6045bde1041a