I think I answer my own question.

I create add onCallOutShow to ClientSideEvents in WebScheduler. Then I have the following code to change the button text.

        function WebScheduler1_OnCallOutShow(controlId) {
            var WebScheduler1 = ISGetObject(controlId);
            document.getElementById("WebScheduler1_CSBEdit").innerHTML = "Edit Reservation";
                        
            return true;
        }

Hi Andi:

This doesn't work. I get "SelectedEvent is null" error message.

Thanks
Michael

I found this article that provides what I need.

http://intersoftpt.wordpress.com/2008/05/08/use-custom-editing-form-in-webschedulernet/