To flip the bus in a distribution equipment, go to
Ribbon: DM Electrical->One-Line Diagram-> 
Flip Bus
Pulldown Menu: DM Electrical->One-Line Diagram->Flip Bus
You will be prompted to select the distribution equipment in which the bus is to be flipped.
Select distribution equipment to flip bus on:
Select the distribution equipment and the bus will be flipped.
For vertical bus bars, if the feeder was being fed to the top of the distribution equipment, it will now be fed to the bottom. If the feeder was being fed to the bottom of the distribution equipment, it will now be fed to the top.