To reverse a route, simply click the arrows next to your driver's name in the Timeline. This will flip the order of your stops in that route backwards.
Note: If you have Live Tracking on your account, you can reverse a route up until your driver departs and checks off their first stop. After the driver checks off their first stop, you will not be able to reverse the route.
If you don't have Live Tracking on your account, then you can reverse a route at any time.