If you have access to the DayPilot.Event object (e) you can change it like this:
// e holds DayPilot.Event object
e.data.areas = [{ html: "Confirmed", right: jsonTags.AreasRight, bottom: jsonTags.AreasBottom }];
dp.events.update(e);
In the context menu item, you can access the source DayPilot.Event object as args.source in onClick:
// ...
onClick: function(args) {
var e = args.source;
e.data.areas = [{ html: "Confirmed", right: jsonTags.AreasRight, bottom: jsonTags.AreasBottom }];
dp.events.update(e);
}
DayPilot.Scheduler.events.update() will redraw the event:
https://api.daypilot.org/daypilot-scheduler-events-update/
Let me know if it doesn't work as expected.