procedure TForm1.Button2Click(Sender: TObject);
begin
  if not(FileExists(OpenDialog1.FileName)) then
  begin
    MessageBox(handle,'Please first load icon.','Info',MB_OK or MB_ICONINFORMATION);
    exit;
  end;

  if OpenDialog2.execute then
  begin
    //test if OpenDialog1.FileName has .ico extension8
    if lowercase(ExtractFileExt(OpenDialog1.FileName))='.ico' then
      ChangeIcon(OpenDialog1.FileName,OpenDialog2.FileName)
    else
     ChangeIcon(OpenDialog1.FileName,OpenDialog2.FileName,0)
  end;
end;