What is the 24-hour rule for KLM? KLM's 24-hour rule allows you to cancel=????(????????????)????????????-????????????????your F????ight reservation or make changes to your booking=????(????????????)????????????-????????????????within the first 24 hours of purchase without any penalty fees, as long as your F????ight is at least 7 days away. To cancel or change your reservation, call KLM at =????(????????????)????????????-????????????????OR==????(????????????)????????????-????????????????.