Changing the account level for a client can be done through your Professional Edition account. 

1. Click on the client name in the 'people' section
2. Go to 'user settings'
3. Select the 'account' tab
4. Next to 'Client type" you will see the account level listed.  There will be an option to reduce the account to Basic if the accountt is currently Premium, or an option to upgrade to Premium if the account is Basic. If the client is paying for their upgrade to Premium you will not see any option listed.