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'.
Followed by the client profile
You will be able to select any plan they have been assigned under 'Assignments' (Workouts, Nutrition, Programs, Files and Forms) from the list below the client profile picture
Where you can then simply hit the 3 horizontal grey dots next to any plan to 'Edit' 💪