Untitled

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Demo Javascript</title>
    <style>
        #th1{
            background-color: lightcoral;
        }
    </style>
</head>
<body>
    <h1 id="th1">This has id th1</h1>
    <button id="ch_btn">Change</button>
    <script>
        function clickBtn(){
            var titleh1 = document.querySelector("#th1");
            titleh1.style.backgroundColor = "lightblue";
        }

        var changebutton = document.querySelector("#ch_btn");
        changebutton.addEventListener('click', clickBtn);
    </script>
</body>
</html>