Log into your paypal account and use the 'Contact Us' at the bottom of the page to tell them you want to downgrade. You can only downgrade once.
Actually, it is not obvious. The Premier and Business accounts shares almost all features and have the same fees. See https://www.paypal.com/us/cgi-bin/webscr?cmd=_display-fees-outside I believe that the OP may no longer be operating his business and wants to keep the paypal account as an individual operating an unregistered business. A Business paypal account requires you to have a registered or incorporated business.