Apply WordPress function on specific woocommerce product categories only

You’re trying to use $product_cat_id even though it isn’t defined anywhere. If you want to know the current category being viewed you need to use is_product_category(6), but you need to use that inside the callback function (ChangeSelectTitle()), because WordPress hasn’t determined whether it’s a category archive yet when it runs functions.php.

function ChangeSelectTitle($woocommerce_product_categories_widget_args) {
    if ( is_product_category(6) ) {
        $woocommerce_product_categories_widget_args['show_option_none'] = __('Select your Device/Brand');
    }

    return $woocommerce_product_categories_widget_args;
}
add_filter('woocommerce_product_categories_widget_dropdown_args', 'ChangeSelectTitle');