Adding Discount to Particular Order Using Order id WooCommerce

//Add below code into functions.php file
function wc_order_add_discount( $order_id, $title, $amount, $tax_class = '' ) {
    $order    = wc_get_order($order_id);
    $subtotal = $order->get_subtotal();
    $item     = new WC_Order_Item_Fee();

    if ( strpos($amount, '%') !== false ) {
        $percentage = (float) str_replace( array('%', ' '), array('', ''), $amount );
        $percentage = $percentage > 100 ? -100 : -$percentage;
        $discount   = $percentage * $subtotal / 100;
    } else {
        $discount = (float) str_replace( ' ', '', $amount );
        $discount = $discount > $subtotal ? -$subtotal : -$discount;
    }

    $item->set_tax_class( $tax_class );
    $item->set_name( $title );
    $item->set_amount( $discount );
    $item->set_total( $discount );

    if ( '0' !== $item->get_tax_class() && 'taxable' === $item->get_tax_status() && wc_tax_enabled() ) {
        $tax_for   = array(
            'country'   => $order->get_shipping_country(),
            'state'     => $order->get_shipping_state(),
            'postcode'  => $order->get_shipping_postcode(),
            'city'      => $order->get_shipping_city(),
            'tax_class' => $item->get_tax_class(),
        );
        $tax_rates = WC_Tax::find_rates( $tax_for );
        $taxes     = WC_Tax::calc_tax( $item->get_total(), $tax_rates, false );
        print_pr($taxes);

        if ( method_exists( $item, 'get_subtotal' ) ) {
            $subtotal_taxes = WC_Tax::calc_tax( $item->get_subtotal(), $tax_rates, false );
            $item->set_taxes( array( 'total' => $taxes, 'subtotal' => $subtotal_taxes ) );
            $item->set_total_tax( array_sum($taxes) );
        } else {
            $item->set_taxes( array( 'total' => $taxes ) );
            $item->set_total_tax( array_sum($taxes) );
        }
        $has_taxes = true;
    } else {
        $item->set_taxes( false );
        $has_taxes = false;
    }
    $item->save();

    $order->add_item( $item );
    $order->calculate_totals( $has_taxes );
    $order->save();
}

Usage:

wc_order_add_discount( ORDER_ID , __("Fixed discount"), DISCOUNT_PRICE );