procedure TForm1.Button1Click(Sender: TObject);
var
  M, N: Word;
begin
  for N := 0 to TabbedNotebook1.Pages.Count - 1 do
    with TabbedNotebook1.pages.Objects[N] as TTabPage do
      for M := 0 to ControlCount - 1 do
        if Controls[M] is TLabel then
          with Controls[M] as TLabel do
            Caption := 'Foo';
end;

procedure TForm1.TabSet1Change(Sender: TObject; NewTab: Integer;
  var AllowChange: Boolean);
begin
  Notebook1.PageIndex := TabSet1.TabIndex;
end;

procedure TForm1.Button2Click(Sender: TObject);
var
  M, N: Word;
begin
  for N := 0 to TabbedNotebook1.Pages.Count - 1 do
    with Notebook1.pages.Objects[N] as TPage do
      for M := 0 to ControlCount - 1 do
        if Controls[M] is TLabel then
          with Controls[M] as TLabel do
            Caption := 'Foo';

end;