using System;

using System.Xml.Linq;

using System.Xml.XPath;

using System.Linq;

​

// Enumerable.ToList(TSource) Method (IEnumerable(TSource)) (System.Linq) <https://msdn.microsoft.com/en-us/library/bb342261(v=vs.110).aspx>

// Extensions.XPathSelectElements Method (XNode, String) (System.Xml.XPath) <https://msdn.microsoft.com/en-us/library/bb342176(v=vs.110).aspx>

// c# - how to change XML node values - Stack Overflow <http://stackoverflow.com/questions/11903552/how-to-change-xml-node-values>

​

public class Program

{

    public static void Main()

    {

    string rawXml = @"<PolicyChangeSet schemaVersion=""2.1"" username="""" description=""""><Attachment name="""" contentType=""""><Description/><Location></Location><SomeElement></SomeElement><SomeDate></SomeDate></Attachment></PolicyChangeSet>";

    Console.WriteLine(rawXml);

    Console.WriteLine("-------------");

    DateTime date = DateTime.Now;

    Console.WriteLine(date.ToString("yyyy-MM-dd"));

    var xml = XDocument.Parse(rawXml);

    ReplaceValue(xml, "//Attachment/SomeElement", "Julius");

    ReplaceValue(xml, "//Attachment/SomeDate", date.ToString("yyyy-MM-dd"));

    Console.WriteLine(xml.ToString());

}

​

// Define other methods and classes here

    static void ReplaceValue(XNode document, string xpath, string replacement)

    {

        var nodes = document.XPathSelectElements(xpath);

        nodes.ToList().ForEach(node => { node.Value = replacement; });

    }

}

​