All existing community event information will be replaced with the new data. To avoid deleting all existing data, retrieve any data you want to retain first, and send it back with this request. See Retrieving community events for more information. Provide an Atom entry document containing the updated Atom document for the community event.
Community event fields that allow update
For single event, you could update all the fields of this event. You could even update a single event into a repeating event. For repeating event, you could update most of the fields.