To insert a node, click the right mouse button anywhere at the graph
drawing and pick one of the "Insert ..." items in the popup menu.

To insert an edge, position the mouse cursor at a node, the edge source,
and drag the rubber string with the middle mouse button to another
node, the edge target.  

To delete a node, first select it by a left mouse button click and then
right mouse button click and pick "Delete selected" in the popup menu.
Multiple nodes can be selected by using a rubber band window with the
mouse or by using a control key and the left mouse button click
simultaneously.  

To edit an edge, select it with the left mouse button click and drag
the edge polyline corners marked be circles. You also can insert and
delete the edge polyline corners and delete the edge itself by right
clicking on a selected edge.

An edge label or a node label can be edited by typing in the textbox
above the viewer control.

An edge label can be dragged by using the left mouse button.

The changes can be undone and re-done by using the toolbar undo-redo 
buttons.