Here is the info in the installation guide.txt of Scroll to the top plugin:
*** You follow these steps to install "Scroll To Top" module
1. Upload "app" folder and their contents to your site (allow overwrite old files).
2. Go to Admin Dashboard >>> Plugins Manager >>> Manager.
3. Open tab "Not Installed Plugins" and find "Scroll To Top" click install.
4. To configure this plugin, please go to: Admin Dashboard >>> Plugins Manager >>> Scroll To Top.
5. Go to Admin Dashboard >>> Clear Global Caches
Just follow exactly the steps above to install.
In case you can't install, please contact us at firstname.lastname@example.org