To edit multiple diffusers, go to
Ribbon: DM HVAC->Diffusers-> 
Edit Multiple Diffusers
Pulldown Menu: DM HVAC->Diffusers->Edit Multiple Diffusers
You will be prompted to select the diffusers to be edited.
Select diffusers to edit:
The Edit Multiple Diffusers dialog box will appear.
Edit Multiple Diffusers Dialog Box
Changes can be made to the diffusers in the same way as when they were inserted. See the Insert Diffuser section for more information.