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

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
            StreamReader ss = new StreamReader(path, Encoding.GetEncoding("Shift_JIS"));
            string text = ss.ReadToEnd();
            ss.Close();
            byte[] data = Encoding.GetEncoding("Shift_JIS").GetBytes(text);

            data[3] = 66;

            //上書き更新
            StreamWriter sw = new StreamWriter(path, false, Encoding.GetEncoding("shift_jis"));
            sw.Write(Encoding.GetEncoding("shift_jis").GetString(data));
            sw.Close();
        }
    }
}