To change a geometry afterwards, you just have to click in the mode "Construction => Storey and building construction" with the left mouse button on the exterior or interior contour you want to change.
After that the area will be highlighted in blue and all previously inserted points will be visible and can be changed, as can be seen in the attached video. New points can be added with a right mouse click in the context menu "Add point".
Please make sure that the result overview must be closed!