Hello @Jana67,
It is totally up to you. When you say "cancelled", do you mean they actually did cancel on the Airbnb system their booking or you mean they arrived the day later?
If the booking is being processed normally then I would ask the guest to make the alteration request on their own and they will receive the refund according to your cancellation standards. If they are nice people that have been good guests, and you feel it's the right thing to do, then through the resolution center offer them a 2 night refund.
If they actually cancelled the booking but arrived anyways, then it's very different, you will not be paid fully and neither the guest nor you are covered by Airbnb anymore as the booking is officially canceled.
Hope this helps. Cheers!