1) Go to the Spark Business Apps marketplace and Login in the top right corner - click here.
2) Enter your User ID & Password on the Login Page of Spark Business Apps.
3) Select My apps
4) Select "Upgrade to paid Edition"
5) Enter the number of Administrators and Mobile Users you want and hit Continue
6) Fill in your Billing Details and select your payment method as below
7) If you selected to Bill to Credit Card you will need to enter your Credit Card details and Make Purchase