To modify a diffuser that is inserted on the drawing, go to
Ribbon: DM HVAC->Diffusers-> 
Query Diffuser
Pulldown Menu: DM HVAC->Diffusers->Query Diffuser
Select a diffuser on the drawing. The information in the database regarding the selected diffuser will be displayed in the Query Diffuser dialog box.
Query Diffuser Dialog Box
Changes can be made to the diffuser in the same way as when it was inserted. See the Insert Diffuser section for more information.