Hi Tony - STL files use a different kind of data than MoI if focused on using for modeling.

MoI uses a type of geometry called "NURBS surfaces". STL files do not contain NURBS surfaces, instead they contain triangle mesh data.

MoI can generate an STL file by taking its NURBS surfaces and dicing them up into little triangles, but it is not easy to take already diced up triangles and make broad smooth NURBS surfaces from those. So because of that using STL with MoI is usually a one way process, you can export to STL but not import it back in again.

If your STL file originally came from a MoI model then you would want to make your adjustment to your original model (usually saved as a 3DM file), then re-export that to a new STL file.

- Michael