All of this happens in the billing portal, reached from Manage subscription on the billing page.
Managing a subscription
Changing plan or screen count, paying for it, getting invoices, and cancelling.
In this section
Change your plan or screen count
Add or remove licenses at any time, and what happens to the money in each direction.
Payment and invoices
Changing the card, fixing the address invoices go to, and finding past ones.
Cancelling
Cancel at any time, keep playing until the period ends, and take your content out first.