using System;

using System.Threading;

using System.Threading.Tasks;

using System.Collections.Generic;

using System.Xml.Linq;

using System.Linq;

​

​

public class C 

{   

    public static void Main()

    {

        XDocument xmlDocument = XDocument.Parse(@"<Students>

    <Student Id='101'>

      <Name>Mark</Name>

      <Gender>Male</Gender>

      <TotalMarks>800</TotalMarks>

    </Student>

    <Student Id='102'>

      <Name>Rosy</Name>

      <Gender>Female</Gender>

      <TotalMarks>900</TotalMarks>

    </Student>

    <Student Id='103'>

      <Name>Pam</Name>

      <Gender>Female</Gender>

      <TotalMarks>850</TotalMarks>

    </Student>

    <Student Id='104'>

      <Name>John</Name>

      <Gender>Male</Gender>

      <TotalMarks>950</TotalMarks>

    </Student>

  <Student Id='105'>

    <Name>Todd</Name>

    <Gender>Male</Gender>

    <TotalMarks>980</TotalMarks>

  </Student>

</Students>");

        xmlDocument.Element("Students")

           .Elements("Student")

           .Where(x => x.Attribute("Id").Value == "101").FirstOrDefault()

           .SetElementValue("TotalMarks", "999");

    }   

}