From their Help Center: Q: How do I upgrade to a Premier or Business account? A: To upgrade to a Premier or Business account, follow the steps below. If you would like to continue to do business under your own name, choose a Premier account. If you would like to do business under a company or group name, choose a Business account. Business accounts come with the ability to give multiple users some level of access to the account, but otherwise offer all the same features as Premier accounts. When you upgrade your account, you will retain the account history of your Personal account. 1. Log in to your PayPal account. 2. Click the Upgrade Account link in the Enhance Account box. 3. Click Upgrade Now. 4. Choose a Business or Premier account. 5. Click Continue. 6. If you chose to upgrade to a Business account, fill in your business information and click Continue. 7. You have now upgraded your account.
Try it anyway - when you click the Upgrade Account link (step 2) there might be an option to go from Premier to Business. Humour me.
Contact them and ask them if they can upgrade it - there's a webform you can use. I've contacted them in the past and they were quite helpful.
Why don't you contact paypal customer service and ask them the way to upgrade your account,i think it is the best option