using System.Xml;

using System;

public class Program

{

    public static void Main()

    {

        try 

        {

            string xml = @"<?xml version=""1.0""?>

                            <Usuarios>

                                <Usuario>

                                    <User_ID>001</User_ID>

                                    <Password>010309</Password>

                                    <Password_Change_Date>00/00/00</Password_Change_Date>

                                    <User_Login>admteste</User_Login>

                                    <User_RG>00000002</User_RG>

                                    <User_Status>Normal</User_Status>

                                    <User_Profile>4</User_Profile>

                                </Usuario>

                                <Usuario>

                                    <User_ID>002</User_ID>

                                    <Password>01234</Password>

                                    <Password_Change_Date>01/10/2019</Password_Change_Date> 

                                    <User_Login>pbteste</User_Login>

                                    <User_RG>00000005</User_RG>

                                    <User_Status>Inicial</User_Status>

                                    <User_Profile>3</User_Profile>

                                </Usuario>  

                             </Usuarios>";

            XmlDocument xmlDoc = new XmlDocument();

            xmlDoc.LoadXml(xml);

            XmlNode noPassword = xmlDoc.SelectSingleNode("/Usuarios/Usuario[User_ID=" + "001" + "]/Password");

            if (noPassword != null)

            {

                noPassword.InnerText = "minha_nova_senha";              

            }                                           

            Console.WriteLine(xmlDoc.OuterXml);

        }

        catch (Exception)

        {

            throw;

        }                       

    }

}