You have three options for doing this:
You can respond from the invitation that appears on your manager's calendar (which is shared with you). Just click anywhere on the event, and then click Yes, Maybe, No, or Remove.
If your manager has given you access to his or her Gmail account, you can also respond from the “Invitation” email in your delegated view of your manager’s Inbox. Just click Yes, Maybe, or No in the Going? section.
Note: Your manager must have personally set up notifications for his or her own calendar in order to receive this email.
If you’ve set up email notifications for your manager’s calendar, you can respond directly from the "New Event" email you receive in your Inbox. Click Yes, Maybe, or No in the Going? section.
If you want to add a note to your response to an event, go to the event details page. You can do this by:
On the event details page, you can add your note to the Going? section near the top of the screen, and click Save. Because not all people choose to get notifications about event responses, you may also want to email the meeting owner with any important messages.
You can quickly view your meeting status without going into the event details page.
Event invitations to which you haven’t yet responded have a reply arrow before the meeting time.
Events with a “Maybe” response
Event invitations to which you’ve responded “Maybe” have a question mark before the meeting time.
Accepted events don’t have a question mark.
Declined events are dimmed, and the event title has a strike through it.
Other Calendar icons explained
When you hover your mouse over Calendar events, you might notice other icons at the top of the event. Below is a list of the icons you will see and a description of what each icon represents.