When you receive an appointment, task, or note, you can accept or decline it. The sender can find your response by checking the item's properties.
If you decline an appointment after its start time, the sender sees the appointment as deleted rather than declined in the appointment’s properties.
In the Calendar, tap the appointment, task, or note that you want to accept or decline.
If the item is set as a recurring event, you are prompted to accept or decline all events or just this event, and given the option to sent comments to the sender.