The Billing section of the admin dashboard offers you a quick look of any upcoming invoice, current usage and subscriber count.
Under the Payments tab you have access to your credit card information where you're free to edit your card and billing location details. On this same page you can add a promo code and change your renewal period.
When changing your renewal period you have the option of yearly or monthly. If you're currently on monthly, you can move to yearly at any time and the day you make the switch will become your new renewal date. If you want to move from yearly to monthly, this switch will not occur until the end of your existing yearly period. This is because you've already prepaid for that yearly service.
Under Invoices, you'll see a list of all available invoices as downloadable PDFs. In additional, you can filter based on year. Learn more about invoices.
The Usage tab will give you update today information on your currently monthly usage period as well as the ability to look up past periods. Usage can be broken down by user/number, country and minutes.