[Help]Replace word in textbox1

How do you replace a word in a textbox with something else. Like this

If TextBox1.Text.Contains("[Hello]")
TextBox1.Replace("[Bye]")

But there is no replace function in textbox1.text.

This will replace all [Hello] with [Bye] .

[php]If TextBox1.Text.Contains("[Hello]") Then
Textbox1.Text.Replace("[Hello]","[Bye]")
End If[/php]

@Water: That will remove all the text and just write "[Bye]". He just wants replacement of [Hello] with [Bye].