To add (or change) the calendar that is displayed on your app, go to Manage School / Calendar and paste in the URL (website link) to your calendar. It does need to be a "public" calendar for your users to access it. Once done, a new tab will show up on your app called "Calendar."
If you are using a Google Calendar and are not sure of the link, then visit this help page and use the instructions under "Share as a link using the Calendar Address".
NOTE - you can also change this at any time.