Modify XML Data at runtime

view the following link this will help you.

http://support.microsoft.com/kb/317666

Also refer this : -

 protected void fillXml()

    {

        DataSet dsbanner = new DataSet();

        dsbanner = bsadv.getBannerStore();

        string s = Server.MapPath("../aspx/aspxBanner.xml");

        dsbanner.WriteXml(s);

        xml();      

    }

    public void xml()

    {

        try

        {

            // Create an XML document instance, and load XML data.

            XmlDocument doc = new XmlDocument();

            string s = Server.MapPath("~/aspx/aspxBanner.xml");

            doc.Load(s);

            XmlNode nodelist = doc.DocumentElement;

            foreach (XmlNode node in nodelist.ChildNodes)

            {

                foreach(XmlNode node1 in node.ChildNodes)

                {

                    if(node1.Name == "sBannerImageName")

                        node1.InnerText = "../admin/banner/" + node1.InnerText;

                }

            }

            doc.PreserveWhitespace = true;

            XmlTextWriter wrtr = new XmlTextWriter(s, Encoding.Unicode);

            doc.WriteTo(wrtr);

            wrtr.Close();

        }

        catch (Exception ee)

        {

        }

   }