For a start you would remove this script (which is at the bottom of the page):
<script src="js/jquery.nicescroll.min.js"></script>
Then you’d need to remove any specific css that the script required. You should be able to find exactly what’s been added in the documentation for the plugin so you will need to check but the main thing is to remove the overflow-y:hidden from the body tag in your style.css
body {
/* overflow-y: hidden; */
letter-spacing: .04em;
word-spacing: .1em;
font-size: 18px;
line-height: 2.3em;
}
That may indeed be all that’s needed but its hard to tell from here