You can add/remove products from a plug after you have created it at any time by visiting your Plug's sheet. In your Plugs page, click on the row corresponding to your plug, or click the 3 dot menu on the right and choose "View".

Choose the Products tab.

Removing products from a plug:

To remove a product from a plug, click the 3 dot menu under the Products tab of your plug's sheet and choose “Detach”.

Adding products to a plug:

If your plug has no product attached, then click the "Attach Products" link at the bottom

In the modal box that appears find the product that you wish to add to your plug and click "Select", then click “Save”. The order in which you select products is the order they will be displayed at checkout.