Scrolling between #anchor links is eased automatically by Webflow. If you also want smooth scrolling with keyboard / mouse wheel, you can use the smoothscroll library.
Using the following custom code in the head of the website:
<script>window.SmoothScrollOptions = { animationTime: 400 }</script>
<script async src="https://cdnjs.cloudflare.com/ajax/libs/smoothscroll/1.4.6/SmoothScroll.min.js" integrity="sha256-Yut1YAlvRzggjdRiTu+X56CcayNzkjwk3p34ZZL0WHk=" crossorigin="anonymous"></script>
Options explained on the smoothscroll project wiki.