You can't edit a workout you've already created. Once a workout is made, it is added to the database for all of our users to use. If someone else decided to perform your workout and you subsequently edited it, it would also edit the workout they've planned for their gym or themselves. That wouldn't be good!
We're working on a way to generate a template from a workout that you can then quickly edit into a completely different workout. It's in the works. Stay tuned!