iCloud 
Reply to an event invitation
When you’re invited to an event, you can reply to the invitation in the invitation email or in the Notifications box in iCloud Calendar.
If you accept the event invitation, it appears in the first calendar listed in the sidebar, but you can move it to a different calendar later if you want.
Reply to an invitation
Do one of the following:
In the invitation email you receive for the event, click Accept, Decline, or Maybe.
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In iCloud Calendar, click the Notification Center button below the calendar. In the notification for the event, click Accept, Decline, or Maybe.
Change your reply
Do one of the following:
In the main calendar window, double-click the event, then click one of the reply buttons.
Open the original invitation email and update your response.