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.

<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="2columns-left" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
        <referenceContainer name="content">
            <referenceBlock class="Magento\Catalog\Block\Product\ListProduct" name="category.products.list" as="product_list">
                <action method="setDefaultDirection">
                    <argument name="dir" xsi:type="string">desc</argument>
                </action>
            </referenceBlock>
        </referenceContainer>
        <move element="category.view.container" destination="content" before="-"/>
    </body>
</page>

Ref: http://magento.stackexchange.com/questions/119642/magento2-default-to-sort-by-price-descending-not-ascending

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