Billing is done at the organization-level.
Click on "Organizations" in the sidebar which will take you to a page containing all of the organizations that you have access to. Once there, click on the organization for which you wish to add, update or remove your billing information. On this page you'll see a section called "Billing". Here you can add, update or remove your billing information.
Note that you won't be able to see or manage your billing information if you've been added to an organization as a member without "owner" status. If you don't see the "Billing" section on your organization's page, contact the owner(s) of the organization.