If you want to deny an appointment then you have to access the Appointments and then click View action. Then you can take the required action by clicking History tab.
Once you click View then you will navigate to that appointment details. In details there is a tab History, please click that tab and appointment history details will display. Now you can deny that appointment by selecting Denied in Appointment Status drop down list and click Add History button. You can also add your message and notify customer by checking the checkbox option Notify Customer via SMS.