If you wish to reset the entire guide, select ‘start workout’ on the guide in question, scroll to the bottom of the page and select ‘reset guide’ a prompt will appear asking if you are sure, if you are, select yes.
If you wish to reset a specific workout, select the week in which the workout is in e.g. Week 1, select the specific completed workout and you will see a ‘Restart Workout’ button. Press it and that particular workout will be reset, keeping the other workouts in the guide their completed state.