You can edit your existing workouts if you want to fix something or if you want to reuse part of it to build a new workout.
Here is a video that will show you how to edit a workout
To edit a workout you first need to click the workout menu:
You will then see the option to edit the workout:
When the workout opens in the Session Designer, you can edit the workout to your satisfaction and then hit save:
In the save-window you will see 2 options: 1) "Save as existing workout" which will replace the first workout with your newly updated version or 2) "New workout" which will create a new workout without saving any changes to your old workout.