From your 'My Hub' page (please see below)
If you hit the 'Menu' icon in the top right...
and select 'Contacts' from the options
You can then select the client's profile
and hit 'Assignments' from the list below the client profile picture and clicking 'Files'
You will then be able to update any files assigned to that client by clicking the 'Assign Files' button at the bottom of the screen.