If you are willing to rename ‘Trip Enquiry’ text displaying in single trip page, please add following given code in functions.php file of your child theme.

function wp_travel_enquiry_text_change($trip_enquiry_text){
    $trip_enquiry_text ='Your Text';
    return $trip_enquiry_text;
}

add_filter('wp_travel_trip_enquiry_popup_link_text','wp_travel_enquiry_text_change');

Also, regarding creating child theme, please refer link.

If you have any queries, please submit it to our Contact page.