This filter changes the string.

Source

File : /inc/helpers.php

Example :

function wp_travel_change_string($localized_strings){
    $localized_strings ['bookings']['start_date'] = 'Departure Date';
    $localized_strings ['bookings']['arrival_date'] = 'Departure Date';
    return $localized_strings;

}
add_filter('wp_travel_strings','wp_travel_change_string');