Edit a workout for a specific client

From your 'My Hub' page (please see below)

On the left hand side of your account, you will see the side menu in which you can select 'Contacts'.

Select a client profile

And click on 'Assignments' from the list below the client profile picture followed by 'Workouts'

Where you can then hit the 3 horizontal grey dots next to any workout to 'Edit Workout'.