You can currently only reschedule runs from your Training Plan via the website.
To reschedule a run, first go to your dashboard. You should see a button that says "Reschedule Runs" in the right-hand corner above your schedule.
Click on the Reschedule Runs button and simply drag any run to a different day.
Once you've moved all runs you'd like to edit, just click Save Changes and you should be good to go!
Keep in mind that dragging a run onto the same date as another run will replace that run.