Add scroll to top button with simple way by using this plugin, very simply to use without having technical knowledge, just install & activate plugin and goto top button will appear, also you will get setting page where you can enable or disable the button.

Easily and safely add a "Scroll to Top" button to your WordPress website.

Show responsive button scroll top on site.

The errnio Scroll to Top plugin adds a free and simple button to any Wordpress mobile site. It appears once the user scrolls down, at the bottom right of the screen.