Refer the following article to know How to add a new Product Modifier set?
1. Click Products on the Navigation Bar and select Products from the drop-down
2. Select the product for which the product modifier is to be added
3. Click on edit to make any changes
4. Scroll to the Modifier Set field and select the product modifier ( Click the checkbox on forced so that after adding the product to the sale, it automatically asks for the modification )
Click Save to save the changes made. Refresh the billing screen to make the changes updated.