In order to change default sort order from ascending to descending in Magento 2, first you need to copy vendor/magento/module-catalog/view/frontend/layout/catalog_category_view.xml to app/design/frontend/{{Vender_Namespace}}/{{Theme_Name}}/Magento_Catalog/layout/catalog_category_view.xml into your theme. Now copy below code to your new xml file.

Thats it. Enjoy Magento 2!! Enjoy Magewallet!!! Ref: http://magento.stackexchange.com/questions/119642/magento2-default-to-sort-by-price-descending-not-ascending