Replace plugin function with a custom function?

For your question how can I replace this function . You don’t need to replace it. The function has a filter on the returns value, not an action. You don’t need to remove it, just filter the result.

So you can create a new function and call it with add_filter('sep_the_event_end_date', 'yourfunction');
Put this in your functions.php file without the add_action plugin _loaded.