You have to call them. We had to do that when, after ordering a movie, we realized that it was cropped, losing a third of the picture. (Unlike other movie PPV services, Verizon alters and crops the movies, which is a rip-off for us). As long as you haven't started to watch the other film, they will apply a credit to your account.
I unintentionally rented a second movie with my first by looking at the tittle. How do I remove it from my account?