If you want to move one vertex of a discharge section to a new location, without moving the other vertex, then use the Stretch Section option.
Select Stretch Section from the toolbar or the Discharge menu.
Use the mouse to select a vertex of a discharge section.
Press Enter or right click and select Done Selection.
There are various ways of defining the stretch. See the Move Vertex option (for boundaries) as the procedure is the same.
Right Click Shortcut
The Stretch Section option is also available as a right click shortcut. If you right click the mouse on a vertex of a discharge section, various Move options will be available in the popup menu. See the Move Vertex option for details, as the procedure is the same.