At any time while you're navigating a route, you can tap the cog button in the lower left hand corner to detour or cancel a route. Tap Detour and select the number of miles on the route to be avoided on the left side, or select a particular road to be avoided on the right side.

You can also detour from the turn-by-turn directions list on the right side of the map screen. Tap any road name or maneuver to detour around it.