Debug.Print Replace("hallo", "a", "q")

Private Sub Form_Load()
 
  Call MsgBox(ReplaceA("hallo", "a", "q"))
 
End Sub
 
 
Private Function ReplaceA(ByVal sValue As String, ByVal sSearch As String, _
  ByVal sReplace As String) As String
Dim i           As Integer
 
  For i = 1 To Len(sValue)
    If Mid(sValue, i, 1) = "a" Then
      Mid(sValue, i, 1) = "q"
    End If
  Next i
  ReplaceA = sValue
 
End Function