In this example you will see how to use an edit sketch to reshape a shared edge. You’ll need to set the edit task to Reshape Edge and turn on snapping to the watershed edges.

The first and last vertex of the sketch should snap to the edge, or you can start the sketch on one side of the edge, cross it where you want the reshaping of the edge to begin, digitize the new shape, and cross the edge again before you finish the sketch. Snapping to the edge is usually simpler.