HI Ashish,
Yes i added the instanceurl.My above code is

HttpClient hc = new HttpClient();

            string consumerkey = "3MVG9Nvmjd9lcjRl5xQxpv_xyGmf8xD1xxozWzj3M2GOgmRau8SN1ElVxZ4Hqnl.5tcWCikg6.iqEVVRI3_R_";
            string consumersecret = "4228649568468639459";
            string concat = String.Concat(password, token);
            HttpContent content = new FormUrlEncodedContent(new Dictionary<string, string>
  {
     {"grant_type","password"},
     {"client_id",consumerkey},
     {"client_secret",consumersecret},
     {"username",username},
     {"password",concat}
   }
);

            HttpResponseMessage obj1 = await hc.PostAsync("https://test.salesforce.com/services/oauth2/token", content);
            string repstring = await obj1.Content.ReadAsStringAsync();

            JObject obj = JObject.Parse(repstring);
            string oauthToken = null;
            string serviceUrl = null;
            string session = null;
            oauthToken = (string)obj["access_token"];
            serviceUrl = (string)obj["instance_url"];
            session = (string)obj["signature"];

HttpClient updateclient = new HttpClient();


            string requestMessage = "<root><name>test</name></root>";
            HttpContent updatecontent = new StringContent(requestMessage, Encoding.UTF8, "application/xml");

            //string requestMessage = "{\"Name\":\"DevForce20\"}";
            //HttpContent updatecontent = new StringContent(requestMessage, Encoding.UTF8, "application/json");

string updateurl = serviceUrl + "/services/data/v25.0/sobjects/Account/";


            //create request message associated with POST verb
            HttpRequestMessage request1 = new HttpRequestMessage(HttpMethod.Put,updateurl);

            //add token to header
            request1.Headers.Add("Authorization", "Bearer " + oauthToken);
         
            //return xml to the caller
            request1.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));
            request1.Content = updatecontent;
       
            HttpResponseMessage response1 = await updateclient.PutAsync(updateurl+"001O000000MPrtsIAD",request1.Content);

            string resultipp = await response1.Content.ReadAsStringAsync();
          

            Response.Write(resultipp);

            return resultipp;