The Reverse One Duct command is used to swap the starting and ending points of a selected duct centerline.
To reverse the direction of a single duct, go to
Ribbon: DM HVAC->Ducts-> 
Reverse One Duct
Pulldown Menu: DM HVAC->Duct Centerlines->Reverse One Duct
You will be prompted to specify the duct for which the direction is to be reversed.
Select duct or label to reverse:
The selected duct will be reversed.