You can either use the automatic plugin installer or your FTP program to upload it to your wp-content/plugins directory the top-level folder. Don't just upload all the php files and put them in /wp-content/plugins/.
Activate the plugin through the 'Plugins' menu in WordPress
Visit your Scroll to Top Options (Options - Scroll to Top)
Configure any options as desired, and then enable the plugin
Requires: 2.9.2 or higher Compatible up to: 2.9.2 Last Updated: 2013-3-12 Downloads: 32,653