I believe the UP button extension has been mentioned, previously. However, may be of interest to point out it has variations available... can be & do more than go up. So, the red circle selects type of button or its function(s); there are 4 options. Showing is (my) having changed from Scroll to Top Only, the default, to Dual arrows. You can scroll (bar) down to see all the choices. AND there is a Save button a bit lower down. Below is a snip of what Dual arrows is which, can go up OR down, to Top OR to Bottom of the page.