Joel:
For the model (3D), you can use Solid Elements Operations to remove part of the object. For the plan (2D) you can open the object (GDL Objects > Open Object...) and change its 2D symbol, or draw over the object in the plan view. You will then need to keep editing the 3D and 2D separately as Solid element Operations only work on the 3D model, not the plan view.
HTH
David