domPractice.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>DOM Practice JS</title>

    <style>
        .container {
            width: 200px;
            height: 200px;
            background-color: #9f2525;
            transition: 1s;
        }

        .grande {
            width: 500px;
            height: 500px;
            background-color:aqua;
        }

    </style>

</head>
<body>
    <button onClick="accion()">Cambio</button>
    <br>
    <div id="box" class="container"></div>

    <script>
        function accion() {
            let box = document.getElementById("box");

            if (box.className == "container"){
                box.className = "container grande";
            }else{
                box.className = "container";
            }
        }
    </script>
</body>
</html>