<Path  Name="PathL2" Fill="Black" Stroke="Blue" Margin="96,171,872,356" RenderTransformOrigin="0.5,0.5" Visibility="Hidden">
                <Path.Data>
                    <GeometryGroup >
                        <LineGeometry    StartPoint = "0,330"  EndPoint = "930, 330" >
                        </LineGeometry>
                        <EllipseGeometry Center="932,330" RadiusX="3" RadiusY="3" />
                    </GeometryGroup>

                </Path.Data>
            </Path>

   var line = ((GeometryGroup) PathL2.Data).Children.OfType<LineGeometry>().FirstOrDefault();
   line.StartPoint = new Point(100, 330);

<LineGeometry x:Name="LG2" StartPoint = "0,330"  EndPoint = "930, 330"></LineGeometry>

LG2.StartPoint = new Point(100, 330);