A line is a list of stops. You do not have to clear it to change one corner.
Drag a stop marker and release it on the street you want. The gaps on either side of that stop are the ones that change, so those are the ones fetched again. The rest of the line stays.
Drag the route stroke itself when the corner you want is between two stops and neither marker is in the right place. On release, a new waypoint is inserted on that stretch. Shift-click a stop to remove it. Delete removes the last point. Z undoes an add.
Clicking near the start after two or more points still closes a loop. The closing gap is just another leg, from the last stop back to the start. Opening the loop again is an undo or a delete of that closing point.
Save when the line is the one you will run. Until then, edit. The map keeps the stops, and only the gaps you touched go back to the router.