Once you've marked a habit as "Formed" or a task as "Finished," you cannot reactivate the habit. However, for tasks, you can undo their completion on the To-Do page by clicking the completion button again. 

After you have confirmed your completion of the habit/task, if you attempt to click into it from the Dream detail page, you will be presented with a greyed out screen of the habit/task page.

If you wish to restart a habit/task, you must manually add the Task or Habit into the Dream again by clicking on the + button at the bottom right corner of the page.