Change Particular line of Multiline textbox in C#

I am unable to Change the specific string of a multiline TextBox.

suppose first line of multiline textbox is "Hello" & second line is "Bye".But when i trying to change the value of second line like below.

textBox1.Lines[1] = "Good bye";

When I saw the result using Debug mode it was not "Good bye".

I also read this MSDN article & this stackoverflow question but can't get the desired answer.