To cancel an appointment, please go to Listing address > Appointments 

This is where you will see all the Approved, Booked and Pending appointments of your listing, so if you need to cancel one of those, you just have to look for the icon right below an approved showing, after clicking you will be asked to provide a reason for the cancellation then submit.

To decline a showing request, please select the REQUEST tab.

Select which appointment request you would like to accept/decline by highlighting the radio button or row and then click Accept/Decline button