Creating basic Hex reading/writing app

Here's some basic code to do that. I'll leave it to you to add any error handling you need:

      using System.IO;

      // add these fields to form
     
      private FileStream fs;
      private int offset = 0x0C; // or whatever
      private string filePath;
 
      // in read button click eventhandler
  
      filePath = "somefile.bin";
      fs = File.Open(filePath, FileMode.Open, FileAccess.Read);
      BinaryReader br = new BinaryReader(fs);
      br.BaseStream.Seek(offset,SeekOrigin.Begin);
      byte b = br.ReadByte();
      br.Close();
      textBox1.Text = b.ToString("X2"); // display in hex

      // after edit in save button click eventhandler    

      byte b = Convert.ToByte(textBox1.Text, 16);
      fs = File.Open(filePath, FileMode.Open, FileAccess.Write);
      BinaryWriter bw = new BinaryWriter(fs);
      bw.Seek(offset,SeekOrigin.Begin);
      bw.Write(b);
      bw.Close();