asensei will always make a recommendation for the Up Next workout, but you might want to choose one yourself to add an additional session to a program you are following, or to run a different warmup or cooldown, or you might just want to try something different today.
To choose a workout, press the Hey asensei [+] button on the Up Next conversation screen, then choose Workouts. This will bring up a list of all the programs you are subscribed to. To enter one of the programs tap on it and a list of available workouts will appear. Simply tap on the workout you wish to use and it will start to download (unless it's already downloaded), then when the blue progress bar reaches the right side of the screen you can tap one more time to run.
Note that if you are following a program and you run a workout from later in the program asensei will skip that workout in your Up Next recommendations. So if you were following Eric Murray's (RE)COMMIT program and tried Empty the Tank which is the last workout in the program asensei wouldn't recommend it to you at the end of the program - you would still be able to run it manually should you wish to.