If you are running fixed schedules, you can do this – from the app go to “Edit Watering Schedule” in “Watering Schedule”. Select your schedule. At the very top of that screen, it should have “Run Schedule” – select that and you should be good to go.
Edited to add: It looks like this should work with all schedules. See http://support.rachio.com/article/148-manually-run-zones-and-schedules