The ‘Event Details’ tab found on the product page can be modified in an upgrade-safe way by overriding the FooEvents template files.
Copy the eventtab.php template found at the following location:
wp-content/plugins/fooevents/templates/eventtab.php
Upload the file to the following location in your theme directory:
wp-content/themes/YOUR_THEME_NAME/fooevents/templates/eventtab.php
Warning: Do not edit these files within the FooEvents plugin itself as they are overwritten during the upgrade process and any customizations will be lost.
Find out more about modifying Fooevents and WooCommerce templates here.