How to use the HttpWebRequest object and the HttpWebResponse object to modify the urn:schemas:mailheader:subject property in Visual C#

using System;
using System.Net;
using System.IO;         

namespace WebDavNET
{
   /// <summary>
   /// Summary description for Class1.
   /// </summary>
   class Class1
   {
      static void Main(string[] args)
      {
         try 
         {
            // TODO: Replace with the URL of an object in Exchange Server
            string sUri = "http://ExchServer/Public/MyFolder/Test.EML";

            System.Uri myUri = new System.Uri(sUri);
            HttpWebRequest HttpWRequest = (HttpWebRequest)WebRequest.Create(myUri);

            string sQuery;
            sQuery = "<?xml version='1.0'?>" + 
               "<a:propertyupdate xmlns:a='DAV:' " + 
               "xmlns:m='urn:schemas:mailheader:'>" + 
               "<a:set><a:prop>" + 
               "<m:subject>" + "ModifiedSubject" + "</m:subject>" +
               "</a:prop></a:set>" + 
               "</a:propertyupdate>";

            // Set Credentials
            // TODO: Replace with appropriate user credential
            NetworkCredential myCred = new NetworkCredential(@"DomainName\UserName", "Password");
            CredentialCache myCredentialCache = new CredentialCache();
            myCredentialCache.Add(myUri, "Basic", myCred);
            HttpWRequest.Credentials = myCredentialCache;
        
            //Uncomment the following statement and comment the previous 4 statements if you 
            //use Integrated Windows authentication
            //httpWRequest.Credentials = CredentialCache.DefaultCredentials

            //Note In Basic type authentication, the username and the password are sent as base64-encoded text, which is 
            //easily decoded. Microsoft recommends that you use Basic over SSL to help protect the username and the password.

            // Set Headers
            HttpWRequest.KeepAlive = false;
            HttpWRequest.Headers.Set("Pragma", "no-cache");
            HttpWRequest.Headers.Set("Translate", "f");
            HttpWRequest.ContentType =  "text/xml";
            HttpWRequest.ContentLength = sQuery.Length;

            //set the request timeout to 5 min.
            HttpWRequest.Timeout = 300000;
            // set the request method
            HttpWRequest.Method = "PROPPATCH";

            // You must store the data in a byte array
            byte[] ByteQuery = System.Text.Encoding.ASCII.GetBytes(sQuery);
            HttpWRequest.ContentLength = ByteQuery.Length;
            Stream QueryStream = HttpWRequest.GetRequestStream();
            // Write the data to be posted to the Request Stream
            QueryStream.Write(ByteQuery,0,ByteQuery.Length);
            QueryStream.Close();

            // Send Request and Get Response
            HttpWebResponse HttpWResponse = (HttpWebResponse)HttpWRequest.GetResponse();
            
            // Get the Status code
            int iStatCode =  (int)HttpWResponse.StatusCode;
            string sStatus = iStatCode.ToString();
            Console.WriteLine("Status Code: {0}", sStatus);
            // Get the request headers
            string sReqHeaders = HttpWRequest.Headers.ToString();
            Console.WriteLine(sReqHeaders);

            // Read the Response Stream
            Stream strm = HttpWResponse.GetResponseStream();
            StreamReader sr = new StreamReader(strm);
            string sText = sr.ReadToEnd();
            Console.WriteLine("Response: {0}", sText);

            // Close Stream
            strm.Close();

            // Clean Up
            myCred = null;
            myCredentialCache = null;
            HttpWRequest = null;
            HttpWResponse = null;
            QueryStream = null;
            strm = null;
            sr = null;      
         }
         catch (Exception e)
         {
            Console.WriteLine("{0} Exception caught.", e);
         }
      }
   }

}