You can edit this in the xml but also after installation
After installation:
1. AdminCP/Products & Hooks/Manage Template Hooks/Edit the "Back to Top"-Template
Change Hook Location from "footer end" to "footer_before_body_end"
Then the button should be visible in forum. But in browser console you'll see a unnessesary loading error for jquery.
2. AdminCP/Styles/Search in Templates/search for "backtotop"/choose template "backtotop"
Delete line 9 without replacement
Code:
<script src='http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js' type='text/javascript'></script>