By default, Staventabs Pro automatically determines the directions of stems. However, you can change the stem direction of all notes in a voice via the Instrument editor.
Let's make the stems of the notes on the bass clef staff point downwards.

-
Choose Instrument > Instrument Editor… (from the Instrument menu at the top of your screen), or press Shift-Command-E
  . - In the Instrument editor, go to the Tracks
section. - Click the Voices row located below the Track · Bass (F4) C heading.
- Using the Stem direction
switcher, change the direction from Automatic to Downwards for the voice 1. - Click Done to apply changes.