If you modify the event in .events.list and call update() it should be updated properly (as all events will be redrawn as part of the update).
See also:
http://api.daypilot.org/daypilot-scheduler-update/
Calling dp.events.update() shoud work as well (and it will redraw only this event). You need to use a DayPilot.Event() object that is initialized using the data object from .events.list (the exact instance). You can get such object by calling .events.find(id) or using new DayPilot.Event(dp.events.list[i]);
See also:
http://doc.daypilot.org/scheduler/client-side-event-api/
Let me know if it still doesn't work.