qualcosa del genere:
codice:
    Private Function replace(ByVal s As String, ByVal c As Char, ByVal position As Integer) As String
        Dim ca() As Char = s.ToCharArray
        ca(position) = c
        Return New String(ca)
    End Function

        Dim a$ = "abcdefg"
        Response.Write(replace(a, "X"c, 2))