When you first subscribe, a payment method is required. You can update this main payment method later if you'd like.
To update your payment method:
Go to Settings.
Click on the Subscription.
Click on the Payment methods on the sidebar.
Add a new payment method by providing the card details or click the PayPal button.
If you want to remove the previous payment method, click on the three dots icon next to it and select the Delete option. This option will only be enabled if you have a backup method already added, or after your subscription is cancelled.