how to add a node to xml file in this case?

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml;

/// <summary>
/// Summary description for xmlOperate
/// </summary>
public class xmlOperate: System.Xml.XmlDocument
{
    private void bindEvent()
    {
        this.NodeChanged += new XmlNodeChangedEventHandler(this.MyNodeChangedEvent);
    }   
   
 public xmlOperate()
 {
        bindEvent();
  
 }

    public xmlOperate(string fileName)
    {
      &nbs ...