Magento 2: Change Upsell text: We found other products you might like!

With Custom Theme:

Override items.phtml in your theme, Copy items.phtml

From:

vendor/magento/module-catalog/view/frontend/templates/product/list/items.phtml

To

app/design/frontend/Vendor/theme/Magento_Catalog/templates/produc‌t/list/items.phtml

Now change

$title = __('We found other products you might like!');

to

$title = __('Customers also viewed');

Flush cache and try.

With Custom Module:

Create catalog_index_index.xml at app/code/Vendor/Module/view/frontend/layout

<?xml version="1.0"?>
<page layout="1column" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
        <referenceBlock name="product.info.upsell">
            <arguments>
                <argument name="template" xsi:type="string">Vendor_Module::product/list/items.phtml</argument>
            </arguments>
        </referenceBlock>
    </body>
</page>

Copy vendor/magento/module-catalog/view/frontend/templates/product/list/items.phtml to app/code/Vendor/Module/view/frontend/templates/product/list/items.phtml

Now change

$title = __('We found other products you might like!');

to

$title = __('Customers also viewed');

Flush cache and try.