Admins have the ability to upgrade accounts to a Business plan through the Organization Settings.
Navigate to the Billing tab at the top of the screen, where you can select Upgrade to Business on the left side of your screen.
Select which subscription plan you would like (Monthly or Yearly).
Enter the number of users, and your credit card information, then click Upgrade.
Your plan should now be upgraded to business, and you will have an invoice viewable in your account under the Billing tab.