Polylines created in NUBIGON and imported as editable CAD can be modified.
Launch the CAD window 
from the sidebar and ensure Viewport editing is enabled.
- Drag components: Hover over an edge or a vertex, then left-click, drag and release.
- Add vertex: Hover over an edge and right-click.
- Delete vertex: Hover over an existing vertex and right-click.
- Close: Drag the first or last vertex onto the other to close an open polyline.
Modify vertex coordinates & edge lengths
- Select a polyline to view its attributes in the Object Inspector.
- Change vertex coordinates and edge lengths as needed.
Coordinates of a 2D polyline cannot be arbitrarily changed. Only changes that maintain the planarity of 2D polylines are allowed.