To reschedule a lesson series, first click on the first lesson in the series that you want to reschedule, and check the update series checkbox:
Next, click on the Schedule tab:
On this tab, check the Update Repetition Schedule checkbox to reschedule the series:
This makes the Repeat Days set of checkboxes appear. Modify these checkboxes to modify the schedule:
Finally, click the update button.
Note there is a second set of checkboxes, Bump Schedule, above. These modify how the lessons react to bumping, not the days they are scheduled.