The short answer: You can't.
Richmond is a one-way course and although it does double-back on itself at certain points, there is no way to turn around on Richmond.
If you want, you can point a mirror at the screen to "ride Richmond backwards", or you can wait until it's a Watopia Island day to ride a different and changeable set of routes where you can turn around, or you can try a few other methods to ride the other course.
Lastly, if that doesn't work, you can just ride solo, with nobody else on the course.
Enjoy the ride!