Here's how I did it on Weebly:
1) Go to http://www.scrolltotop.com/
2) Pick which "scroll up" tool you want
3) Copy the code (highlight text, then right click and choose "copy")
5) Click on "Settings"
6) Click on SEO
7) Find the "Footer Code" box
9) Click "Save"
10) Update your site by hitting "Publish"
*NOTE: Your button will only appear after you have started scrolling down. I didn't know this when I first did it and thought I did something wrong :( Luckily I soon figured it out once I started scrolling.