Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<head>
<style>
.example {
    color: red;
    padding: 5px;
    width: 150px;
    font-size: 15px;
}

.newClass {
    color: blue;
    padding: 15px;
    width: 250px;
    font-size: 18px;
    background-color: white;
}
</style>
</head>
<body>

<p>Click the button to change the value of the button element's second attribute.</p>

<button onclick="myFunction()" class="example">Try it</button>

<script>
function myFunction() {
    var a = document.getElementsByTagName("BUTTON")[0];
    a.attributes[1].value = "newClass";
}
</script>

</body>
</html>