Updating a client’s Files

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'.

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.