We understand that sometimes you have new payment source and wish to use that in the next payment.
If you are using Paypal, go to your PayPal dashboard and add a different funding source. Be sure to remove all old source of funding.
Alternatively, you can first cancel your recurring payment, and start a new subscription with Piktochart (not recommended).
Link on how to cancel recurring payment: https://piktochart.zendesk.com/hc/en-us/articles/200678974-How-do-I-cancel-a-recurring-payment-
If you used Braintree to make the payment, the only way to change the payment source currently is to re-subscribe. We are working on making this possible in Piktochart very soon.