This is something that can search easily on google ... No need to ask the community !
However see Here's The Article for you ! I would suggest a mod member to take care of closing this post.
I don't know which site builder you're using, but this works at least in V7 for one page at a time: Choose 'button' from the menu. Click on the button to bring up its menu. Click on the icon that looks like a gear. Under 'label', type 'top'. Under 'link', choose from the 'destination' menu the page you are on. When in preview, test this; you should return to the top of the page you are on.