Miky9:
For a Line, mirror the Line on its center point.
For a Polyline, I had some success adding a Line to the end of the PolyLine, then using the Magic Wand on these 2 parts to create a new Polyline that is the opposite direction, then stretching the extra segment of the new Polyline back to its original end to eliminate it.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David