If you want to change your credit card, you need to click on the profile icon in the top right corner and then choose the menu Subscriptions.
It will lead you to a new page where you can see your current subscriptions, their status, the card you’re currently using, etc.
On top you can see Change default credit card. There you need to enter your Card number, month/year of expiration and the CVC number from the back. Once you’ve filled in the required details, you need to click Apply.