Michael, are you trying to connect to the Extract that your previously created and running now to update the data? If so, Tableau Prep doesn't have an Extract refresh option yet. However, when you first created the flow you might have also saved a .tfl file (it contains all the tables and manipulations you've made), try refreshing your data using .tfl file and create the extract again. That should work.
Also, feel free to upvote this idea,