using System;

using System.Xml;

using System.Xml.Serialization;

using System.IO;

​

public class Program

{

    public static void Main()

    {

        string xml = @"<?xml version=""1.0"" encoding=""utf-8""?>

            <Invoice xmlns:cac=""urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"" 

                     xmlns:cbc=""urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2"" 

                     xmlns:ext=""urn:oasis:names:specification:ubl:schema:xsd:CommonExtensionComponents-2"" 

                     xmlns=""urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"">

                <cbc:ProfileID>reporting:1.0</cbc:ProfileID>

                <cac:AdditionalDocumentReference>

                    <cbc:ID>PIH</cbc:ID>

                    <cac:Attachment>

                        <cbc:EmbeddedDocumentBinaryObject mimeCode=""text/plain"">NWZlY2ViNjZmZmM4NmYzOGQ5NTI3ODZjNmQ2OTZjNzljMmRiYzIzOWRkNGU5MWI0NjcyOWQ3M2EyN2ZiNTdlOQ==</cbc:EmbeddedDocumentBinaryObject>

                    </cac:Attachment>

                </cac:AdditionalDocumentReference>

            </Invoice>";

        var Xdocument = new XmlDocument();

        //Xdocument.Load("data.xml");

        Xdocument.LoadXml(xml);

​

        var nsm = new XmlNamespaceManager(Xdocument.NameTable);

        nsm.AddNamespace("inv", "urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"); 

        nsm.AddNamespace("cbc", "urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2");

        //nsm.("urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"); 

        var el = Xdocument.DocumentElement.SelectNodes("/inv:Invoice/cbc:ProfileID", nsm);

​

        Console.WriteLine(el.Item(0).InnerText);

        el.Item(0).InnerText = "abc";

        Console.WriteLine();

​

        var stringWriter = new StringWriter(new System.Text.StringBuilder());

        var xmlTextWriter = new XmlTextWriter(stringWriter) {Formatting = Formatting.Indented};

        Xdocument.Save(xmlTextWriter);

        Console.WriteLine(stringWriter.ToString());

    }

}