You can change your subscription in your Settings, with the Billing tab.
In the Billing tab you are able to update :
(A) your subscription to go from Pro to Entreprise (or vice versa)
(B) the number of licences. For instance, if you wish to add users, check on the Billing tab that you have enough licences, if not you can add somme using the dedicated button pencil-shapped. If you wish to lower your licences, first disable the concerned users (users management), then update the number of licences with the dedicated button (B)