There is also an “EDIT” button under the design thumbnail. Some themes might hide this or make the text very small, you may need to use CSS to boost the size or look of that link. That link allows people to edit their design and replace the one in the cart automatically, rather than creating a second design and adding that to cart too, and having to then manually remove the first one.
Also note we have an Add-On in our Marketplace for Changing Options in Cart. If you do go straight to cart, that is another way for users to edit the selections they made on the product page after the design app UX completes and they (by default) land in the cart. This add-on lets people change those options in dropdown menus in the cart and save their changes to update the cart totals etc when they save any changes.