renp

A simple WordPress plugin which adds a “scroll to top” button to the bottom corner of the page if the page has been scrolled down. It is compatible with all major browsers and should work with all themes, including 
responsive and high resolution themes.
It is possible to configure the plugin in the admin area for choosing between light or dark colour schemes and size of the button as well.
If your theme doesn’t already contain this feature, you can simply add it by means of this plugin.

Scroll to Top Button

Displays a button in the bottom right corner for smooth scrolling to the top of the page.

kubiq

Replace basic website scrolling “effect” with nice smooth scroll using mousewheel, touchpad or keyboard arrows
You can play with settings and test it here: 
<a href="https://demo.tastewp.com/mousewheel-smooth-scroll" rel="nofollow ugc">https://demo.tastewp.com/mousewheel-smooth-scroll</a>

MouseWheel Smooth Scroll

Smooth scrolling experience, with mousewheel, touchpad or keyboard

Scroll to Top Button vs MouseWheel Smooth Scroll