jQuery addClass example

Change class name on click in jQuery

HTML

<a id="size_boot">Кнопка</a>
<div class="strelka">Контент</div>

CSS

.strelka {
  background: tomato;
}
.transparent {
  opacity: 0;
}

JavaScript

$('#size_boot').on("click", function(){
  $('.strelka').toggleClass('transparent')
})