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 decides to use the same workout, and you were to edit that workout, it will 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 edit into a completely different workout. It's in the works. Stay tuned!