Navigate over to the products page and view the table of products.
Find the product you wish to delete assets for, and click the grey arrow at beginning of the desired product row, then click the right variant of the product. This will load you into a 'Assets' page. From here you can hit the red 'Delete' button at the end of each asset file row.
A popup confirmation will be presented before any deleting is actioned.
Please note, the deleting of a asset file is irreversible and we wont have a backup copy so be careful.