To update your credit card details, go to Settings then Billing and click on the Update Plan button.
You'll be presented with a box that contains a small button that can be switched between 'Per Month' and 'Per Year'. if you're currently on a monthly plan, you can update that to an annual plan you will save 15%.
Selected Per Year or Per Month and then click Continue.
Next you'll be asked to enter your new credit card details and then click Submit.