Hello,

You can modify the footer view file themes/default/views/footer.php as you need.

Please add the following code before </body>

<button onclick="(function goToTop() { $('html, body').animate({scrollTop:0}, '250'); })()" style="position: fixed; bottom: 20px; right: 20px; padding: 2px 10px; background: #fff; border: none; border-radius: 4px">↑</button>

Thank you