How to make an event style as selected on mouse click?

You can change the event background color using onEventClick event handler:

dp.onEventClick = function(args) {
  var e = args.e;
  e.data.backColor = "blue";
  dp.events.update(e);
};

There is also a built-in event selection mechanism but that reverts the styling when you deselect the event:
https://doc.daypilot.org/scheduler/event-selecting/

Dear Dan,

The onEventClick is not firing while clicking on the event. And also I need to restrict the multiple event selection and the selection must be removed on clicking on the any other location of the daypilot.