using System;

using System.Xml;

using System.Xml.Linq;

using System.Xml.XPath;

public class Program

{

    public static void Main()

    {

        var xdoc = XDocument.Parse(@"<?xml version=""1.0"" encoding=""utf-16""?>

        <Document>

  <Engineering version=""V17"" />

  <DocumentInfo> 

  </DocumentInfo>

  <SW.Blocks.FB ID=""0"">

    <AttributeList>     

      <Interface><Sections></Sections></Interface>

      <MemoryLayout>Optimized</MemoryLayout>

      <MemoryReserve>100</MemoryReserve>

      <Name>EM00_CM01_Warp1</Name>

      <Number>31650</Number>

      <ProgrammingLanguage>LAD</ProgrammingLanguage>

      <SetENOAutomatically>false</SetENOAutomatically>

    </AttributeList>

    <ObjectList>    

      <SW.Blocks.CompileUnit ID=""4"" CompositionName=""CompileUnits"">

        <AttributeList>

          <NetworkSource>

            <FlgNet xmlns=""http://www.siemens.com/automation/Openness/SW/NetworkSource/FlgNet/v4"">

                <Parts>    

                  <Access Scope=""GlobalVariable"" UId=""27"">

                    <Symbol>

                      <Component Name=""HMIAxisCtrl_Interface"" />

                      <Component Name=""axesInterface"" AccessModifier=""Array"">

                        <Access Scope=""LiteralConstant"">

                          <Constant>

                            <ConstantType>DInt</ConstantType>

                            <ConstantValue>0</ConstantValue>

                          </Constant>

                        </Access>

                      </Component>

                    </Symbol>

                  </Access>   

                </Parts>

            </FlgNet>

          </NetworkSource>         

        </AttributeList>     

      </SW.Blocks.CompileUnit>       

    </ObjectList>

  </SW.Blocks.FB>

</Document>

  ");

        var nm = new XmlNamespaceManager(new NameTable());

        nm.AddNamespace("sm", "http://www.siemens.com/automation/Openness/SW/NetworkSource/FlgNet/v4");

        var node = xdoc.XPathSelectElement(@"//sm:Access[@Scope=""LiteralConstant""]/sm:Constant/sm:ConstantValue", nm);

​

        node.Value = "Something Else";

        Console.WriteLine(xdoc.ToString());

    }

}