If you are on a prepaid plan, there is nothing to update. Just enter your new details when you next want to make a payment.

If you are on a rolling subscription, you should see a button on your Billing page that says Update Credit Card:

If you click that button, that should take you to our provider's subscription page, from where you can click Edit Subscription:

and then choose Update Payment Method or Update Address to go ahead with the changes you want: