Unfortunately, you cannot change the payment method dynamically at this time.
To use another method of payment or update your credit card details, you need to cancel your current subscription
if you have one, wait until it expires and make another purchase with the new payment method.
You can cancel the subscription on the Account page here: https://windscribe.com/myaccount
The reason for this is that we don't actually keep your payment details on file. This is done in order to store less info about our users so all the payments and payment methods are handled by trusted third party processors such as Paypal and Stripe.