I'd like to modify the text of an email template. How can I do it?
In order to edit the email, please go to Marketing → Email Templates and create a new template. To associate the new template with the back-in-stock alerts, you'd need to load the existing template. Please select Magento_ProductAlert → Stock Alert from the list and click the Load Template button.
Then, specify the name and other required properties, edit the Template Content to your requirement and save.
After the template is ready, you'd need to set it as default in the settings. Please navigate to Stores → Configuration → Catalog → Catalog → Product Alerts to define the new template: