Airlines typically have to follow multiple sets of rules- first there will be a government rulebook based on where the airline is headquartered and where the airline flies. Second will be a document called a 'contract of carriage' which spells out a lot on how the airline operates including how passengers on cancelled or substantially delayed flights will be accommodated to get them to their final destination in a reasonably timely manner. (Or spell out what happens in terms of refunds or flight credits if the cancellation or delay is so severe that the passenger no longer wants to make a trip)

For a KLM flight operating within European Union rules the passenger protections are relatively generous. They can't make a seat of a different flight appear out of thin air, but they will do their best to get you there as soon as possible. The airline might also have to pay you compensation for a cancelled or delayed EU flight if certain conditions are met. A little info on what's known as EU 261 rules:

https://en.wikipedia.org/wiki/Flight...ation_261/2004

If you're inexperienced with booking plane travel, do look up some flight alternatives online and then call the airline or ask one of their airport desks to rebook you onto that new flight rather than rebooking on your own. It's not hard to do, but might be best for your own peace of mind, and because your desired free-of-charge rebooking might need a 'nudge' from an employee's computer to get you onto the right plane. (Also- If you're trying to do something like rebook a complex routing during a mass cancellation due to weather, a good airline employee can force you into seats onto a flight that the computer system doesn't automatically offer a passenger rebooking on their own)