To edit the copy right go to Library/Xenforo/ and open the file Application.php file with notepad ++ and on line 1430 you will see this.
Code:
return '<a href="https://xenforo.com" class="concealed">Forum software by XenForo™ <span>©2010-2016 XenForo Ltd.</span></a>';
edit to what you want!
Code:
return '<a href="https://domain.com" class="concealed">Forum software by hidden; <span>©2010-2016 Domain.com</span></a>';
Enjoy!