In the Inventory Section click under the Inventory Item for select the product.
Once you select your required Product ,product will appear at your home screen.
Now you will find the “Modifiers” check box, under the area of Product type.
To action this change clicks on the “Attach Modifiers” now select your required modifier to add with the product.
After Complete click on "save" to proceed.
Now do click on "update " that will show on product manager section.