Untitled

<!DOCTYPE html>
<html>
    <head>
        <title> Prva zadaca</title>
    </head>

    <body>
        <p id="counter"></p>

        <h3 id="naslov" class="test" style="color:red;">OVA E NASLOV</h3>
\


        <br><br><br><br><br><br>

        <button onclick="changeHeader()">KLIKNI</button>


        <script>
            var header = document.getElementsByClassName("test");

            console.log(header)


            function changeHeader(){

                console.log(header);
                header.addClass("");
                header.innerHTML = "MODELIRAN NASLOV";
            }
        </script>
    </body>
</html>