If a change is made to an event at your school that you have entered manually, you can make changes to that event, even if it's published.
Click the 'Calendar' link in the main navigation.
Navigate to the event that you need to edit in your calendar and click on it.
The pop-up will have two buttons - edit and delete. Click the edit button to change the details of your event. Make any required changes and then click 'Save' and your changes will publish immediately.