<button class="btn">click</button>
<div class="contet">
  <div class="content__item"> 1
  </div>
  <div class="content__item content__item_active"> 2
  </div>
  <div class="content__item"> 3
  </div>
  <div class="content__item"> 4
  </div>
</div>

.content__item {
  color: red;
}

.content__item_active {
  color: blue;
}

$(".btn").on("click", function() {
  $(".contet")
    .find(".content__item")
    .each(function(index, element) {
      $(this).toggleClass("content__item_active");
    });
});