In the Plan page (with the list of tasks) click on a task and use the calendars to set the start and end dates.
Alternatively from the Plan page rotate the phone screen to landscape. The display will change to schedule view. From here you can:
- scroll the calendar
- drag a task to a new date
- drag from the start or end of the task to change the duration.
Rotating the phone to portrait will return to the Plan page. Note that if autoschedule is set on, the tasks will be moved to conform with the autoschedule dates.