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 WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string path = Application.StartupPath + @"\test01.DAT";

            //byte配列へ変換
            FileStream fsr = new FileStream( path,FileMode.Open,FileAccess.Read);
            byte[] data = new byte[fsr.Length];
            fsr.Read(data, 0, data.Length);
            fsr.Close();
            fsr.Dispose();
            data[3] = 66;
            
            //上書き更新
            FileStream fsw = new FileStream(path, FileMode.Create, FileAccess.Write);
            fsw.Write(data, 0, data.Length);
            fsw.Close();
            fsw.Dispose();
        }
    }
}