There is no automatic way to downgrade a business account to a personal account, you have to contact customer services and they will do that for you.
Click help / contact bottom left of Paypal pages for options available for your country.
Or have you considered contacting Customer Service via Facebook or Twitter? You can send them a personal message from their facebook or twitter pages. It's: https://www.facebook.com/PayPal and @AskPayPal for Twitter.
Advice is voluntary. Kudos / Solution appreciated.