When a user is at the bottom of a long page, scroll to top button helps them to navigate back to the top of the page with a single click. This saves the user from scrolling many times to reach the top of the page.
Go to Sysetm > Settings > Preferences > Scroll to top.
Check the box of Enable Scroll to top. You can even change the position of the button.
Click on Save button on the right top corner.