Find I-frame byte on MPEG4 file with C#

this is my code. i'm try to find i-frame bits and do XOR processing.

private void button4_Click(object sender, EventArgs e)
{
FileStream fs = new FileStream(textBox1.Text, FileMode.Open);
FileStream fscreate = new FileStream(textBox2.Text, FileMode.Create);
long size = new FileInfo(textBox1.Text).Length;

byte[] mybyte = new byte[size];
Console.WriteLine("start read");
for (int i = 0; i < size; i++)
{
mybyte[i] = (byte) fs.ReadByte();
// if mybyte[i] is i-frame byte. do XOR;
}

fscreate.Write(mybyte, 0, mybyte.Length);

fs.Dispose();
fscreate.Dispose();
}

I want to make changes the pixel color by making changes the bytes data of each frame.