What is the 24-hour rule for KLM? You can change or cancel your 1-833-460-1838 reservation during the first 24 hours after purchase without being charged anything if your flight is at least seven days away. . KLM's 24-hour regulation. Call KLM at ☎️????????-????????????-????????????-????????????????????☎️ OR=☎️????????-????????????-????????????-????????????????????☎️ to modify or cancel your reservation.