Trying to contact air bnb myself, heard you can do it by twitter if thats any help, let me know if you find out as I need to ask them about overcharging me.
You can't. A host should never cancel unless an absolute emergency. I did a mistake once when I was new, and was duly punished, I rang Air but nothing doing. I doubt they have softened on this point, and you will have to wear it and be very careful which buttons you click!