using System;

using System.Xml.Linq;

​

public static class Program

{

    public static void Main(string[] args)

    {

        const string xml =

            @"<S>

 <NPC>

    <NP> A </NP>

    <NPA> B </NPA>

 </NPC>

 <NPD>

     C

 </NPD>

</S>";

​

        var doc = XDocument.Parse(xml);

​

        foreach (var element in doc.Descendants())

        {

            if (element.Name.LocalName.StartsWith("NP"))

            {

                element.Name = "NP";

            }

        }

​

        Console.WriteLine(doc);

​

    }

}