Go to the Account -> Billing and click on the Change Plan button. You can use this to upgrade to a paid plan, or switch between plans.

formcrafts-billing-page

Our plans are prorated so you only pay for the time you have used them. Example, you signed up for our $30 plan. 20 days later you decided to upgrade to the $100 plan. In this case you will get a credit for the un-used portion of the $30 plan ($10 credit) and you will be charged only $33.33 for the 10 days you spend on the $100 plan, which would be reflected on your next invoice.

Same is also true for additional paid users added on your account.