Because a Bezier is defined by its control points position - its shape can only be modified by dragging an control point.

Click the Modify button ( ) to activate the Modify function.

Click on the track. The control points will appear

Left-Click a control point and Left-Drag it to move it.

Repeat for another end point if desired

Once you are happy, hit the spacebar or Enter, or to cancel hit Escape.

Note: A Bezier's end point will only be modifyable if it is not connected. Both internal control points can be modified even if the track is connected.