You can add or remove payments methods on a plug by visiting your Plug's infopage. Visit your "Plugs" page, click the 3 dots menu on the right and select the "View Page" option. Alternatively you can click on the table row.
Scroll and find the "Payment Methods" sections and you will be able to enable and disabled the payment gateways you like for your shopping cart.
Please note that the available payment gateways for each shopping cart depend on the product you are selling.