C# Change File Extension

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
 
namespace RenameExt
{
    public partial class Form1 : Form
    {
        string[] files;
         
       
        public Form1()
        {
            InitializeComponent();
        }
 
         
 
        private void button1_Click(object sender, EventArgs e)
        {
            DialogResult result = folderBrowserDialog1.ShowDialog();
            if (result == DialogResult.OK)
            {
                //
                // The user selected a folder and pressed the OK button.
                // We print the number of files found.
                //
              files = Directory.GetFiles(folderBrowserDialog1.SelectedPath);
     
              foreach (string file in files)
              {
 
                  richTextBox1.AppendText(file.ToString() + Environment.NewLine);
                   
              }
 
            }
 
 
 
        }
 
        private void button2_Click(object sender, EventArgs e)
        {
            foreach (string file in files)
            {
                File.Move(file, Path.ChangeExtension(file, textBox1.Text));
 
            }
            files = Directory.GetFiles(folderBrowserDialog1.SelectedPath);
 
            richTextBox1.AppendText("You have changed following files "+Environment.NewLine);
            foreach (string file in files)
            {
 
                richTextBox1.AppendText(file.ToString() + Environment.NewLine);
 
            }
 
        }
 
        private void button3_Click(object sender, EventArgs e)
        {
            foreach (string file in files)
            {
               File.Move(file,file.Substring(0,file.Length-4));
              
 
            }
 
            files = Directory.GetFiles(folderBrowserDialog1.SelectedPath);
 
            richTextBox1.AppendText("You have changed following files " + Environment.NewLine);
            foreach (string file in files)
            {
 
                richTextBox1.AppendText(file.ToString() + Environment.NewLine);
 
            }
        }
 
        
    }
}