A very simple solutions would be to add an anchor tag to the top of your page using two code blocks at the top and bottom of your page.
In the first code block you would paste:
At the bottom you would add a second code block for the button:
<a href="#scroll-to-top">Back to top</a>
Style to your needs and you are good to go for a very basic button. Please note it will only take your browser to the tag. On my site the header is a bit tall and was not visible when going back to the top of the site.
You can read this more in depth article on the topic: HTML Anchor Tag Tutorial