To change a product 'Permalinks', from the store dashboard go to:
> Products > All Products

From within this page find the 'Product' you'd like to 'Edit' by either scrolling through or searching for it via the search box in the top right:

Once you've found the product you'd like to edit click 'Edit':

At the very top underneath the product title you'll find the 'Permalink':

Click 'Edit' to alter it and then click 'OK'.
(*Don't forget to click 'Update' after making alterations*)