How to write the contents of a textbox to a hex file

[sonic91]

I have managed to make my program read and display my selected hex line as below:

                BinaryReader br = new BinaryReader(File.OpenRead(ofd.FileName));
                string speed = null;
                for (int i = 0x83; i <= 0x8f; i++)
                {
                    br.BaseStream.Position = i;
                    speed += br.ReadByte().ToString("X2");
                }
                br.Close();
 
 
                textBox1.Text = (speed);
 

This will display what i need to know in my textbox e.g. "153f4e6a"


but if i need to change whats in the textbox to e.g. "153ee563". I don't know how to write it back to the file i know its wrong but i thought something like this.

                BinaryWriter bw = new BinaryWriter(File.OpenWrite(ofd.FileName));
                for (int i = 0x83; i <= 0x8f; i++)
                {
                    bw.BaseStream.Position = i;
                    bw.Write (0xTextbox1.text );
                }
                bw.Close();
 

But i don't know the correct way of doing it even tho it could be really easy