Example: I want to replace "a" with "A" in Textbox1 and put the replaced string into Textbox2.
You can't use it directly like this.
Text2.Text = Replace(Text1.Text, "a", "A")
You must make use of variable, like this.
Dim TempVar As String
TempVar = Replace(Text1.Text, "a", "A")
Text2.Text = TempVar
Then the code will work. More info here.