You can make changes to your subscription by going to your «Profile», then «Prices and subscriptions» and then clicking on «Manage subscription». A new «Manage subscription» window then opens. Under «Payment methods», you can add a credit card and then change your subscription as desired via «Edit subscription».
Please note that an upgrade is possible at any time but a downgrade is only possible at the end of the contract. Information on your subscription is found in your «Profile» under «Prices and subscriptions».