Display Groups can always be sorted by dragging-and-dropping the up/down arrows on the left-hand edge of the Display Groups window. Sorting sessions, however, is not quite as obvious.
To sort the sessions within a Display Group, first click on the name of the Display Group on the left. This will filter the list of sessions so that only the sessions assigned to that Display Group are shown. When that's done, sorting arrows will appear on the session list, and the sessions can be dragged-and-dropped into any order you wish.
The session order that's defined here will be reflected on the Event Registration page when end users sign up for the event.