ช่วยด้วยคร๊าฟ มีปัญหาในการ แก้ไข และ ลบ ไฟล์ .Text ใน C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO;

namespace TestEditText
{
    public partial class frmMain : Form
    {
        public frmMain()
        {
            InitializeComponent();
        }

        // ปุ่ม Open
        private void btOpen_Click(object sender, EventArgs e)
        {
            using (OpenFileDialog openFileDialog = new OpenFileDialog())
            {
                openFileDialog.Title = "Select Text File";
                openFileDialog.Multiselect = false;
                openFileDialog.Filter = "Text File (*txt)|*.txt";

                if (openFileDialog.ShowDialog() == System.Windows.Forms.DialogResult.OK)
                {
                    this.txtPath.Text = openFileDialog.FileName;
                    this.btSave.Enabled = true;
                    OpenTextFile(openFileDialog.FileName);
                }
            }
        }

        // ปุ่ม Save
        private void btSave_Click(object sender, EventArgs e)
        {
            SaveTextFile(this.txtPath.Text);
            MessageBox.Show("Save OK", this.Text, MessageBoxButtons.OK);
        }

        // ปุ่ม Close
        private void btClose_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void OpenTextFile(string path)
        {
            using (StreamReader sr = new StreamReader(path, Encoding.Default))
            {
                this.txtEdit.Text = sr.ReadToEnd();
                sr.Close();
            }
        }

        private void SaveTextFile(string path)
        {
            using (StreamWriter sw = new StreamWriter(path, false, Encoding.Default))
            {
                sw.Write(this.txtEdit.Text);
                sw.Close();
            }
        }
    }
}