I've found a road segment where the horse has a very difficult time. I noticed that the NavMesh is entirely underneath the road -- as in doesn't appear on the first eye (normal view), requires the second eye (transparent view) on the toolbar.

But worse, two vertices in the center of the area are much lower than the others. When viewed from the side, it looks like a huge cliff. The horse is having a dikeens of a time getting over this cliff. Obviously, there's a lower widget "F". Where's the raise widget?

The same way as you place/move objects: select the vertices, hold down the Z-key and move your mouse with left button pressed.
This moves the vertices in Z-direction (up/down)

alt-g doesn't work for navmeshes though, and that's a BAD thing.