Add code to your child theme’s
functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Avoid adding custom code directly to your parent theme’s
functions.php file as this will be wiped entirely when you update the theme.
You could also change the “Completed” status to be another order status, like “Processing”
For more flexibility, such as auto-completing only virtual orders, you can use Order status control.