Click your name in the top-right corner and select Settings → Account Details. From there, open the Payment Info sub-tab.
Here, you can view your current payment method and billing address. If you need to update your card, you can do so at any time from this page.
The card listed here is the card that will be billed for your Deep Lawn subscription.
When updating your card, make sure you also update the billing address to match the address registered with your card. Your billing address is used to determine your sales tax rate.