The Update Isometric command is used to update an isometric diagram to reflect changes that have been made on the plan view.
To update an existing isometric diagram, go to
Ribbon: DM Plumbing->Isometrics-> 
Update Isometric
Pulldown Menu: DM Plumbing->Isometrics->Update Isometric
You will be prompted to identify the isometric diagram to be updated.
Select isometric to update:
The selected isometric diagram will be updated to reflect the latest version of the pipe design on the plan view.