You can’t make them all flat at present because the software is not designed specifically to allow that. Hopefully you will not find it too arduous to make all beams flat, which you can do by way of:
- Edit > Select All.
- Edit > Filter > Notes and Chords.
- If you have tuplets in your project: Edit > Filter > Deselect Only, Edit > Filter > Tuplets.
- In the Properties panel, set Beam direction to Flat.