// @nuget: HtmlAgilityPack

using System;

using HtmlAgilityPack;

​

public class Program

{

    public static void Main()

    {

        var html = @"<TR>

        <TD value='1' class=texte width='50%'>

            <DIV align=right>Name :<B> </B></DIV>

        </TD>

        <p id='someid'>hellowwww</p>

        <TD value='2' width='50%'>

            <INPUT class=box data-tooltype='psFileList' value=John maxLength=16 size=16 name=user_name/>

            <INPUT class=box value=Tony maxLength=16 size=16 name=user_name><TD value='2A' ><INPUT value='Bill'/></TD></INPUT>

            <INPUT class=box value=Jams maxLength=16 size=16 name=user_name/>

            <TD value='2B' >

                <INPUT class=box value=Johny maxLength=16 size=16 name=user_name/>

                <INPUT class=box value=Tonky maxLength=16 size=12 name=user_name/>

                <INPUT class=box value=Jammy maxLength=16 size=16 name=user_name/>

            </TD>

        </TD>

        </TR>";

        var htmlDoc = new HtmlDocument();

        htmlDoc.LoadHtml(html);

        var htmlNode = htmlDoc.DocumentNode.SelectSingleNode("//p[@id='someid']");

        htmlNode.AppendChild(htmlDoc.CreateElement("<b>heyyyyy</b>"));

        if (htmlNode != null)

            Console.WriteLine(htmlDoc.DocumentNode.InnerHtml);

    }

}

​