Edit Complex Model
You can access this tool from the following:
The Edit Complex Terrain Model tool is similar in functionality to the Create Complex Terrain Models, but utilizes previously defined complex models. When the tool is selected, the dialog list box is populated with the complex terrain models in or referenced to the current *.dgn file.
Start the command.
Select the existing complex terrain.
The merge box is presented which can be edited by adding or removing component terrains or change merge order or change merge method.
OK to update the terrain.
Create Complex Terrain Model
Merging or Appending Terrain Models