WP Travel plugin provides the feature regarding renaming placeholder ‘TREKKING‘ using filter in itinerary search section(HOMEPAGE) as well as in Search widget as per your requirement.

You can rename placeholder ‘TREKKING‘ by adding following given code in functions.php file of your child theme.

function wp_travel_customize_search_placeholder($placeholder) {
    return 'Placeholder';
}
add_filter( 'wp_travel_search_placeholder', 'wp_travel_customize_search_placeholder' );

For detail assistance regarding creating child theme, please refer link.

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