@Megan405
Hi Megan, many of us have found that where an alteration is required for an existing reservation (apart from sending a special offer) it is always best to get customer support to make the changes. There are too many variables and the automated system will in many instances make unrequired changes.
In this instance I would contact Airbnb in Aukland on +64 4 4880 888, explain what has happened, tell them it was the guest who required trip to be shortened, and get them to sort it out for you!
Cheers......Rob