Demo accounts automatically deactivate after 30-days but it may be that your client needs an additional 30-days (or more), if this is so you can do this manually at any time.
Simply log in to your Main Frame and this time select 'VIEW ACCOUNTS':
Navigate to the account you require and select 'ACTIONS' from the dialogue box:
you can now select the 'EXTEND TRIAL' button to extend the to the subscription required. The trial period for your client will now be re-set and all records/users invited or created during the initial trial will become active again.