How to move up or down between plans from the Subscription page: upgrades start straight away, downgrades wait until the end of the period you have already paid for, plus the few things that can hold a change up.
How to keep your payment card up to date, view and download your invoices, and clear anything that is outstanding from the billing area.
A friendly guide to how your DMARCER bill is worked out from the domains you manage, split into Standard and Parked licences, and how to view, check and (if you use a PSA) push those counts.