To do this, you'd need to run your package again, then re-publish to server. I'd expect that future versions of Prep will allow you to automatically update a data source on the server, but that's not available yet. In the meantime, you can use Windows Task Scheduler to schedule a refresh. See the "Refresh output files from the command line" in the online help: Save and Share Tableau Prep Work
Following up on this one. Let us know how we can help further.