Tap a Task or Dream to open its detail view. 

Note the Due date option on the page. Click on the Due date section to select a date, and select a desired due date for the Task or Dream.

To delete Due date, simply click the 'X' button to the right of the due date.

Additional information:

When you use the Pre-made dream feature, the due date for Dream is automatically set from the starting date according to the predicted duration.