We have a secure portal for you to manage all the credit card info and subscription details.
Please login to your account at http://howmanyextension.com
Click on Account
Click on Update billing details
You can update your payment information two ways. First is clicking "Payment Methods" on this screen and updating there and the second is clicking on your plan name and then following the steps below.
Click "Billing History" to download past copies of your invoices.
Then you will see a screen similar to this one:
You can cancel your subscription on this page.