Notes written above the middle of the staff usually are written with the stems downward, notes on or below the middle with the stems upward. If a group of notes is beamed together, stem direction would typically be chosen according to whether most of the notes were above or below the middle. This is largely an aesthetic decision, and you can set the stem directions as you wish. Select a note or notes and click a stem direction button to alter the passage. That also sets the default stem direction for new notes. The current choice is highlighted.