From your 'My Hub' page (please see below)
If you go to the 'Menu' icon in the top right...
then select 'Contacts' from the options
Select a client profile
and click on 'Assignments' from the list below the client profile picture followed by 'Nutrition Plan'
If you hit the 3 horizontal grey dots to the right of any nutrition plan and select 'Edit'
You will see the assigned plan along with 'Macronutrients' which you will be able to edit.
Just remember to click 'Save Plan' once changes have been made.