изменить текст на кнопке

function zapret() { 
  var btn = document.getElementById('submitButton').disabled = true; 
 
  return false; 
}

<form class="form-horizontal form_add text-center" onsubmit="zapret()" method="post" action="" enctype="multipart/form-data"> 
  <div class="form-group text-center"> 
    <input id="myFile" name="myFile" type="file" accept=".zip,.rar" required> 
  </div> 
  <div class="form-group text-center"> 
    <button type="submit" id="submitButton" class="btn btn-success">Отправить</button> 
  </div> 
</form>