String Replace

Question

Hi,Below is my program to replace the string A to Z, B to Y .. . ..... Z to A and so on

PrivateSub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

Dim sample As String
sample = TextBox1.Text
Dim letter As String = "AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz"
Dim letter1 As String = "ZzYyXxWwVvUuTtSsRrQqPpOoNnMmLlKkJjIiHhGgFfEeDdCcBbAa"        
For Each s As String In sample
Dim i As Integer            
For i = 0 To letter.Length - 1                
If s = letter(i) Then                    
TextBox2.Text += letter1(i)                
End If            
Next
Next 

End Sub

Here i want the program, that should have only one for loop and inside the loop should not use the if conditions. Need the optimized code. Can anyone help me please