To create a superset you must first navigate into an exercise within a routine. From this screen you should tap Add Exercise to Group. Either select an existing exercise or create a new one by typing the exercise name into the search bar, then selecting the exercise from the section named Create New Exercise.

Within the Exercise Group Editor tap the Reorder button from the navigation bar. Here you may reorder exercises by tapping and holding the 3 lines to the right of the exercise name and dragging the exercise to where you would like it. Or you may delete the exercise by tapping the red delete icon and confirming the deletion by tapping the Delete button.