using System;

using System.Linq;

using System.Xml;

using System.Xml.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        string xml = @"<?xml version=""1.0""?>

        <PurchaseOrders>

          <PurchaseOrder PurchaseOrderNumber=""99503"" OrderDate=""1999-10-20"">

            <Address Type=""Shipping"">

              <Name>Ellen Adams</Name>

              <Street>123 Maple Street</Street>

              <City>Mill Valley</City>

              <State>CA</State>

              <Zip>10999</Zip>

              <Country>USA</Country>

            </Address>

            <Address Type=""Billing"">

              <Name>Tai Yee</Name>

              <Street>8 Oak Avenue</Street>

              <City>Old Town</City>

              <State>PA</State>

              <Zip>95819</Zip>

              <Country>USA</Country>

            </Address>

          </PurchaseOrder>

        </PurchaseOrders>";

    XDocument xdoc = XDocument.Parse(xml);

    //xdoc.Descendants("PurchaseOrder").Remove();

​

    xdoc.Descendants("PurchaseOrder").Single().ReplaceWith(new XElement("AnnualOfferOfCoverageCd", 100));

    Console.WriteLine(xdoc);     

    }

}