Try to understand this code so that u will get some idea 
how to change the data in the existing xml file.

using System;
using System.Xml;
using System.Text;

namespace ConsoleApplication1
{
   class Class1
   {
      [STAThread]
      static void Main(string[] args)
      {
         try
         {
            // Create an XML document instance, and load 
XML data.
            XmlDocument doc  = new XmlDocument();
            doc.Load
("Q317666.xml");                      // This code assumes 
that the XML file is in the same folder.

            // I. Modification
            // 1. Increment all of the Book Id attribute 
values by 100.
            XmlNodeList nodeList = doc.SelectNodes
("//Book");
            foreach (XmlNode node in nodeList)
               node.Attributes["Id"].Value = (Int32.Parse
(node.Attributes["Id"].Value) + 100).ToString();

            // 2. Change the book titles to uppercase 
letters.
            foreach (XmlNode node in nodeList)
               node.FirstChild.InnerText = 
(node.FirstChild.InnerText).ToUpper();

            // 3. Modify the XML declaration instruction to 
have Unicode encoding.
            XmlDeclaration decl = (XmlDeclaration) 
doc.FirstChild;
            decl.Encoding = "UTF-16";

            // II. Addition
            // 1. Create a new Book element.
            XmlElement newElem = doc.CreateElement("Book");

            // Add the Id attribute.
            XmlAttribute newAttr = doc.CreateAttribute
("Id");
            newAttr.Value = "103";
            newElem.Attributes.Append(newAttr);

            // Create the child nodes. This code 
demonstrates various ways to add them.
            newElem.InnerXml 
= "<Title></Title><Author></Author>";
            XmlText txtNode = doc.CreateTextNode("A BRIEF 
HISTORY OF TIME");
            newElem.FirstChild.AppendChild(txtNode);
            newElem.AppendChild(doc.CreateWhitespace
("\r\n")); // Linefeed
            newElem["Author"].InnerText = "Stephen Hawking";

            // 2.  Add the new element to the end of the 
book list.
            doc.DocumentElement.AppendChild(newElem);

            // III. Deletion
            // 1. Remove the Genre nodes from Book elements.
            foreach (XmlNode node in nodeList)
               node.RemoveChild(node.SelectSingleNode
("Genre"));

            // Display the output in Debug window.
            System.Diagnostics.Debug.Write("{0}\n", 
doc.OuterXml);

            // 2. Save the modified XML to a file in 
Unicode format.
            doc.PreserveWhitespace = true;
            XmlTextWriter wrtr = new XmlTextWriter
("Q317666_Out.xml", Encoding.Unicode);
            doc.WriteTo(wrtr);
            wrtr.Close();
         }
         catch(XmlException xmlEx)        // Handle the Xml 
exceptions here.         
         {
            Console.WriteLine("{0}", xmlEx.Message);
         }
         catch(Exception ex)              // Handle the 
generic exceptions. here
         {
            Console.WriteLine("{0}", ex.Message);
         }
      }
   }
}