Hello,

First, please update to the latest version 1.0.86
Secon: copy and paste this code into functions.php file of your child-theme:

function change_country_label($label){

	$label = array(
		'name'                       => __( 'Cities',  'boxtheme' ),
		'singular_name'              => __( 'City',  'boxtheme' ),
		'search_items'               => __( 'Search Cities', 'boxtheme' ),
		'popular_items'              => __( 'Popular Cities', 'boxtheme' ),
		'all_items'                  => __( 'All Cities', 'boxtheme' ),
		'parent_item'                => null,
		'parent_item_colon'          => null,
		'edit_item'                  => __( 'Edit City', 'boxtheme' ),
		'update_item'                => __( 'Update City', 'boxtheme' ),
		'add_new_item'               => __( 'Add New City', 'boxtheme' ),
		'new_item_name'              => __( 'New City Name', 'boxtheme' ),
		'separate_items_with_commas' => __( 'Separate Cities with commas', 'boxtheme' ),
		'add_or_remove_items'        => __( 'Add or remove Cities', 'boxtheme' ),
		'choose_from_most_used'      => __( 'Choose from the most used Cities', 'boxtheme' ),
		'not_found'                  => __( 'No Cities found.', 'boxtheme' ),
		'menu_name'                  => __( 'Cities', 'boxtheme' ),
	);
	$result['label'] = $label;
	$result['slug'] = 'city';
	return (object) $result;
}
add_filter('tax_country_args','change_country_label', 99 );

Screenshot:
https://i.imgur.com/FmiFhrT.png