using System;

using System.Linq;

using System.Xml;                   

using System.Xml.Linq;

using System.Collections; 

using System.Collections.Generic;

using System.Data;

using System.Data.DataSetExtensions;

using System.Data.Linq;

using System.Text;

using System.Text.RegularExpressions;

using System.Xml.Serialization; 

using System.IO;

using System.Globalization;

using System.Diagnostics;

public class Program

{

    public static void Main()

    {

        string input = @"<EmployeeResults><MainArea>

    <CreationDateTime>2016-06-28T06:10:51.5215523Z</CreationDateTime>

  </MainArea>

  <SubArea>

    <Show>

      <ID>TEST1</ID>

    </Show>

    <ProductionPerformance>

      <ID>Fabrication_ERP_MES_DEM_1-A</ID>

      <ProductionResponse>

        <ID>123</ID>        

        <StartTime>0001-01-01T00:00:00Z</StartTime>

        <EndTime>0001-01-01T00:00:00Z</EndTime>

        <EmployeeResponse>

          <ID>LotEmployeeResponse</ID>          

          <ActualStartTime>2016-06-28T05:58:41.673Z</ActualStartTime>

          <ActualEndTime>0001-01-01T00:00:00Z</ActualEndTime>            

            <Quantity>

              <QuantityString>1</QuantityString>

            </Quantity>          

          <TagName1>

            <ID>Test1</ID>

          </TagName1>

          <TagName2>

            <ID>Test2</ID>

          </TagName2>

        </EmployeeResponse>

      </ProductionResponse>

    </ProductionPerformance>

  </SubArea>

</EmployeeResults>"; 

        XDocument doc= XDocument.Parse(input);

        doc.Descendants("TagName1").Remove();

        doc.Descendants("EmployeeResponse")

           .ElementAtOrDefault(0)

           .Add(new XElement("TagName3", new XElement("ID", "Test3")));

        doc.ToString().Dump();

    }

}