Choose your movie by navigating on the screen (or you can check online if your particular redbox location has the movie you want) . . . then right before checkout where you swipe your credit card, look at the bottom of the screen for "enter a code." Then enter this code:
Return it by tomorrow night 9pm at ANY redbox location and it's completely free - no charge! I've done this many times. The problem I now have is finding a decent movie to rent! Enjoy!