When you have an active subscription and want to upgrade / downgrade your subscription or change the used payment method - please log in Centrium account owner account and go to 

billing settings page here: http://app.centriumcrm.com/#settings/billing

On this page you have three available actions for managing your subscription:

1) "Change your plan" - to adjust the subscription user count / file storage quota.

2) "Change payment method" -  to switch to different payment  method, keeping current subscription setup.

3) "Cancel subscription" - disable billing for Centrium.