Just click on the section line and then drag the end points to suit.
I had to fiddle with the lines once the "dots/grips" appear on the arrowheads and finally got the lines to move in and out, but it doesn't seem very intuitive. Its easy moving the arrows left and right but moving up and down isn't as easy unless I am missing something. Thanks.
Ok, got it finally. You have to click on the intersection of the horiz. and vert. lines and while holding down the left mouse button, drag the point downward or upward.