Dim arrTxt(1000) As String ' сохраняет каждый шаг
Dim bUndoMode As Boolean ' переключение режима
Dim intCount As Integer ' счётчик Undo
Dim strBuffer As String ' сохраняет текст для Redo

Private Sub Form_Load()
mnuRedo.Enabled = False
End Sub

Private Sub mnuRedo_Click()
  bUndoMode = True
  intCount = intCount + 1
  On Error Resume Next
  strBuffer = Text1.Text
  Text1.Text = arrTxt(intCount)
  bUndoMode = False
End Sub

Private Sub mnuUndo_Click()
  If intCount = 0 Then Exit Sub
  bUndoMode = True
  intCount = intCount - 1
  On Error Resume Next
  strBuffer = Text1.Text
  Text1.Text = arrTxt(intCount)
  bUndoMode = False
  mnuRedo.Enabled = True
End Sub

Private Sub Text1_Change()
If Not bUndoMode Then
    intCount = intCount + 1
    arrTxt(intCount) = Text1.Text
  End If
End Sub