kliknutí na elem

$("section article ul li").click(function() {
  $(this).addClass("aktive").siblings().removeClass("aktive");
});

article section ul li {
    position: relative;
    list-style-type: none;
    color: #FFFFFF;
    background-color: #00BBFF;
    margin-top: 2px;    
    top: 32px;
    width: 99px;
    padding: 15px 5px;
    font-size: 23px;
    font-family: Georgia;
    text-align: center;
    transition-duration: 0.5s;
  }
  
.aktive {
    top: 0px;
  }

article section ul {
    position: relative;
    top: 135px;
    left: 57px;
    width: 99px;
    z-index: 1;
    cursor: default;
  }

<article id="znalosti" class="znalosti">
                        <header><h2 class="znalosti_n">Znalosti</h2></header>
                                <section>
                                    <ul>
                                        <li>...</li>
                                        <li>...</li>
                                        <li>...</li>
                                        <li>...</li>
                                    </ul>