You can add/remove products from a plug after you have created it at any time by visiting your Plug's infopage. In your Plugs page click the 3 dot menus on the right and choose the "View Page".
Removing products from a plug:
To remove a product from a plug, click the "X" icon next to the product's name.
Adding products to a plug:
If your plug has no product attached, then click the "Attach a Product" link next to the "What's for sale" section.
In the modal box that appears find the product that you wish to add to your plug and click "Attach". The product will be automatically added to your plug.