To edit multiple ducts, go to
Ribbon: DM HVAC->Ducts-> 
Edit Multiple Ducts
Pulldown Menu: DM HVAC->Duct Centerlines->Edit Multiple Ducts
You will be prompted to select the ducts to be edited.
Select ducts to edit:
Duct Settings Dialog Box
Changes can be made to the ducts in the same way as when they were inserted or queried. See the Query Duct section for more information.