If you want to change an appointment, the easiest way to do this is to click on the Calendar and then on the appointment you want to change. There you can click on Change booking details. You have the option to change/cancel one appointment or change/cancel all appointments with this customer from there.
Please note! Either changing or cancelling something will not send a notification to your customer. We very strongly recommend only changing or cancelling something after you have discussed this with your customer first.