When you are writing content with two voices sharing the same staff, Flat put the stems of one voice upward, and the stems of the other voice are facing downward.

Two voices with opposed stems

Sometime the stems direction is not the more appropriate:

Two voices with stems on the wrong direction

We just added a tool that comes handy in this kind of situations: you can now swap the voices assigned to the selection. To do so, make a range selection, then click on the voices button. Finally, click on the Swap voice button.

The process of swapping the voices of notes on the selected range

I hope this tool will make the composition process easier for you!