To upgrade your free Volunteer Scheduler Pro account to a paid subscription, go to the Subscription pane, scroll to view the upgrade options, and click the "Select" button for either the Premium or the Team edition:
(If you are already on a paid subscription, there will be an "Upgrade" button in the Subscription pane you can click on to upgrade further and/or add more administrator seats.)
On the next pages, select the number of administrator seats you will need (view-only administrators do not count toward this total), the number of months you will be paying for, and enter your organization's billing information, then click "Purchase" to complete the upgrade.