Adding Calendars
Open your Calendar with the keyboard shortcut option + C on Mac (or alt + C on Windows).
Click "Calendars" on the top of the task list panel.
Click "+ Add Account".
Add your calendar.
Removing Calendars
Open your Calendar with the keyboard shortcut option + C on Mac (or alt + C on Windows).
Click "Calendars" on the top of the task list panel.
Under "My calendars", hover your mouse over the calendar you want to remove.
Click the 3 dot icon to the right of the email, and remove your calendar.