No, as long as the booking is still going on, it should be fairly simple: just go to your messages or confirmation, click on "cancel or change reservation", and proceed with the change. As long as it falls within the host's availability or settings, ( like you can't change a three day stay to a one day stay if host has a two day minumum) it should work out fine, then the other side has to approve, that's all. I think it's easier to extend than to shorten a stay : shortening means a refund to the guest will be processed, lengthening the stay means a new charge.