You will need to include the JavaScript file simple-scroll-to-up.js (or the minified version simple-scroll-to-up.min.js) and the css file simple-scroll-to-up.css (simple-scroll-to-up.min.css).
<link rel="stylesheet" type="text/css" href="css/simple-scroll-to-up.min.css" />
<script src="js/simple-scroll-to-up.min.js"></script>