using System;

using System.Linq;

using System.Xml.Linq;

​

public class Program

{

    public static void Main()

    {

        const string xml =

            "<?xml version='1.0' encoding='UTF-8' standalone='no'?><root><lastBuildDate>Thu, 13 Apr 2017</lastBuildDate></root>";

​

        var doc = XDocument.Parse(xml);

​

        var lastBuildDate = doc.Descendants("lastBuildDate").Single();

​

        var lastBuildDateAsDateTime = (DateTime) lastBuildDate;

​

        lastBuildDate.Value = "new value here";

​

        Console.WriteLine(doc);

    }

}