Tettra admins can update your team's credit card on file or your billing email address via the Team settings->Billing page:
If you have a card on file, you should see a blue 'Update' button next to your card. If you do not yet have a card on file, adding a card will subscribe you to Tettra.
Updating your Subscription Plan Or Billing Cycle
If you are on one of our standard subscription plans you can update your plan or billing cycle (monthly or annual):
If you need more users on your account, just add them. User bundles are added (and removed) automatically when a team goes over (or drops under) the user limit on our current Growing, Scaling and Sustaining plans. For teams who are on older plans, we have to add bundles manually.
For more information on accessing your invoices, check out this related page.