You can add/remove products from a plug after you have created it at any time by visiting your "Plugs" page.
Click on the name of the product (or the number of products) on the corresponding table row of the plug you wish to modify.
Removing products from a plug:
To remove a product from a plug, in the modal box that appears click the last icon on the correspoding row of the product you wish to detach. The product will automatically be removed from that plug.
Adding products to a plug:
On a plug that has no products click the "0 products" text on the corresponding table row of the plug you wish to modify.
In the modal box that appears find the product that you wish to add to your plug and click the last icon. The product will be automatically added to your plug.