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 where you can select 'Contacts'.

Select a client profile

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

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