You can add or remove payments methods on a plug by visiting your "Plugs" page. Simply click on the payment method text link (marked in red below) of the correspoding plug you wish to update.
Inside the modal window that opens you will be able to update your plug's accepted payment methods.
*Note that only online payment methods that you have already linked to your plug&paid account will be available and shown inside the modal window. You can link other online payment gateways (e.g. Stripe or Bitpay) to your account under the "Settings" page.
Do not forget to hit the "Update Payment Methods" button to save your changes.