Value of a specific line of RichTextBox not changing

I am trying to do very simple thing which should work but unfortunately its not working.

I have a RichTextBox component on my Winform. I am trying to change text value of some lines of the RichTextBox but its not changing the value. Here is my code:

    private void button1_Click(object sender, EventArgs e)
    {
        for(int i=0; i < richTextBox1.Lines.Length; i++)
        {
            if(richTextBox1.Lines[i] == "ok")
            {
                richTextBox1.Lines[i] = "Done";
            }
        }
    }

I put break point and I notice that it executes

richTextBox1.Lines[i] = "Done";

But it does not change the value at all. Can anyone explain it? Why its not modifying the value? Is there way to change/modify the value as per Line?

Thanks and Regards