If you don't have a calendar yet, you will need to first create one.
Creating a calendar
1. Navigate to Google Calendar.
2. Click "Create".
3. Name the calendar and fill in the details.
4. Either make your calendar public, or make sure that the calendar is shared with the
members of your site.
5. Click "Save".
Congratulations, you have created a calendar. You will need to return to calendar to add events. If you shared it with other Google accounts they will automatically see the calendar in their calendar view and can also add events.
Return to your site
Replace the existing calendar.
1. Click the edit button.
2. Click on the calendar image.
3. Click the "Properties" link.
4. Click the "Change" button.
5. Choose a calendar from the list and click "Select".
6. Choose the options you want for your Calendar and click Save.
7. When you're finished editing your page, click "Save" and your Calendar will appear.