To link a lower level model to a higher level model select the object at the top and use the right hand button to select Link as shown to the right. 

 

Note: This window can also be opened from the main toolbar Design Menu "Objects/Hierarchy"

 

Choosing Link in the menu shown above will open the explorer window so the lower level model can be specified as shown to the right. Once a lower model is linked any Flag name changed on the lower model will be reflected to the top when the lower level model is saved.

 

Ports added to the top model will appear as shown below (Bloomed) if a corresponding IO Flag for that port is not on the lower level model. These must be resolved and corrected. (Either removed or corrected at the lower level)