Often, you'll need to manually delete WP Super Cache's cached files from your server via FTP. Once they have been deleted and the cache purged (including the device cache), if the rejected user agents have been set up correctly, the caching plugin should no longer serve the desktop theme to mobile devices.
To configure your caching plugin not to cache mobile pages nor to serve cached pages when mobile devices are detected, enter the user agents found under Advanced Options > Custom User-Agents to the "rejected user agents" areas in your caching plugin's settings.