You can add the button using a block element.
1. Go to appearance > elements, and create a new block element.
2. Choose hookas element type, and choose before_footer as hook name.
3. Add the button and style it in the content area, add smooth-scroll class to the button:
https://wordpress.com/support/wordpress-editor/adding-additional-css-classes-to-blocks/
4. Set the button’s URL to#top.
5. Go to customizer > general, and enable smooth scroll feature.