Where can I change my subscription and billing/payment information?

You can use the Subscription tab in your customer portal to change your subscription and billing information.

Here in the subscription tab of the customer portal you can find out everything you need about your subscription.

You can edit your billing and payment information and change your subscription.

If you upgrade your subscription you may be billed additional costs, depending on the upgrade.
If you downgrade your subscription you will get credits towards your next billing.