In order to drag and drop tasks in your projects, you'll first need to make sure your project is in GTD format. To do this, go into your Task View and then your Project List. Tap the edit option on the view selector. Tap the project you'd like to change the format for. Choose Next Action Handling: Sequential (GTD)

Once your Project is in GTD, you can drag and drop tasks within that project by tapping the small square to the right of the Task and moving it where you'd like.