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 red unlink icon on the correspoding row of the product you wish to detach.
Adding products to a plug:
In the modal box that appears click the "Attach Products" button.
Choose the products you want to attach to your plug and click the "Attach Products" button.
That's it, your plug will be automatically updated with your new selections.