// Change"city" checkout billing and shipping fields to a dropdown
add_filter( 'woocommerce_checkout_fields' , 'override_checkout_city_fields' );
function override_checkout_city_fields( $fields ) {

    // Define here in the array your desired cities (Here an example of cities)
    $option_cities = array(
         '' => __( 'Select your city' ),
        'Karachi' => 'Karachi',
        'Lahore' => 'Lahore',
        'Faisalabad' => 'Faisalabad',
        'Rawalpindi' => 'Rawalpindi',
        'Gujranwala' => 'Gujranwala',
        'Peshawar' => 'Peshawar',
        'Multan' => 'Multan',
        'Hyderabad' => 'Hyderabad',
        'Islamabad' => 'Islamabad'
    );

    $fields['billing']['billing_city']['type'] = 'select';
    $fields['billing']['billing_city']['options'] = $option_cities;
    $fields['shipping']['shipping_city']['type'] = 'select';
    $fields['shipping']['shipping_city']['options'] = $option_cities;

    return $fields;
}