Can you reverse normals in Reality Composer pro? or do you need to do it externally?

are there any examples of doing something to a mesh with either SceneKit or Model I/O? I am trying to recreate a process that I am doing in blender, finding the lowest vertices and deleting them and then finding the highest vertices and deleting them, but I am having a hard time finding ANY examples of doing this type of work in Swift and SceneKit or Model I/O.. thanks for any help, Dale