Upgrading is easy. If you have a free account, you will see features that are disabled, but include a link to upgrade to a Business Account, which will take you to the Administration page, Billing section. Optionally, you can go directly to the Administration page. From the Administration tab, Billing section, select the option to upgrade and enter your credit card information and promotional code if you have one.
If you would like to upgrade to an Enterprise Account, please contact firstname.lastname@example.org.