On the Route Planning screen, before you start navigating, you can add waypoints by tapping on the “Add stop” button. You can select location on map by tapping on it, or you can search for the address. When you are selecting location on map, the blue marker will appear, which can be moved around the map, so you can select your waypoint precisely.
You can then adjust the order of all the selected points on your route including the starting point and destination, by tapping on the 2 lines over to the right side of these points and dragging them up or down.