First, ensure your teacher list is updated by clicking on the Go To menu and selecting Teachers. Here, add/remove teachers as needed.
After that, click on the display name in the upper-right corner of your account and select Account.
Here, you'll see an option for Renew Subscription under the "Subscription and Payments" heading. This will allow you to choose the number of teachers you wish to renew and how far into the future you wish to renew for.
On this same page, you can also choose how to receive your invoice. You may also pay the invoice on this page with a credit card.