For Wholesale plugin, in WooCommerce order admin UI, add an action to toggle order between retail and wholesale.

class-wholesale-order-ui.php

<?php
/**
 * Add an action on the WooCommerce orders page to toggle between wholesale and retail orders.
 *
 * Implemented:
 * options 'wholesale'|'retail'
 * `$order->add_meta_data('_wwpp_order_type', 'wholesale', true );`
 *
 * GPL 2+
 */


use WC_Order;
use WWP_Wholesale_Roles;

/**
 * @link https://www.skyverge.com/blog/add-woocommerce-custom-order-actions/
 *
 * Class Wholesale_Order_UI
 * @package BrianHenryIE\WP_GetEnhanced_Shop\WooCommerce_wholesale_prices
 */
class Wholesale_Order_UI {

	const BH_WP_TOGGLE_ORDER_TO_WHOLESALE_ACTION = 'bh_wp_toggle_order_to_wholesale';
	const BH_WP_TOGGLE_ORDER_TO_RETAIL_ACTION    = 'bh_wp_toggle_order_to_retail';

	/**
	 * Add "Set to wholesale order" and "Set to retail order" to order actions in admin UI order edit page.
	 * If neither has already been set, both options appear. If one has been set, the opposite is available to
	 * toggle to.
	 *
	 * @hooked woocommerce_order_actions
	 * @see class-wc-meta-box-order-actions.php
	 * @see WC_Meta_Box_Order_Actions::output()
	 *
	 * @param string[] $actions Associative array of action_id:Action Name for the WooCommerce order admin view actions.
	 * @return string[]
	 */
	public function add_admin_ui_order_action( $actions ): array {

		global $theorder;

		/**
		 * This should never be empty because it is set in WC_Meta_Box_Order_Actions::output() which calls this filter.
		 *
		 * @var \WC_Order $theorder The order currently being viewed.
		 */

		$current_wholesale_order_type = $theorder->get_meta( '_wwpp_order_type' );

		if ( 'wholesale' !== $current_wholesale_order_type ) {

			$actions[ self::BH_WP_TOGGLE_ORDER_TO_WHOLESALE_ACTION ] = 'Set to wholesale order';
		}

		if ( 'retail' !== $current_wholesale_order_type ) {
			$actions[ self::BH_WP_TOGGLE_ORDER_TO_RETAIL_ACTION ] = 'Set to retail order';
		}

		return $actions;
	}

	/**
	 * Action called when shop_manager selects 'Set to wholesale order' in admin order actions drop-down.
	 *
	 * @hooked  woocommerce_order_action_bh_wp_toggle_order_to_wholesale
	 *
	 * @see \WWPP_WC_Order::add_order_type_meta_to_wc_orders()
	 *
	 * @param \WC_Order $order The order to update.
	 */
	public function set_order_to_wholesale( $order ) {
		$this->set_order_to( $order, 'wholesale' );
	}

	/**
	 * Action called when shop_manager selects 'Set to retail order' in admin order actions drop-down.
	 *
	 * @hooked  woocommerce_order_action_bh_wp_toggle_order_to_retail
	 *
	 * @see \WWPP_WC_Order::add_order_type_meta_to_wc_orders()
	 *
	 * @param \WC_Order $order The order to update.
	 */
	public function set_order_to_retail( $order ) {
		$this->set_order_to( $order, 'retail' );
	}

	/**
	 * A common function to update the order with the new wwpp type and the user role.
	 *
	 * @param WC_Order $order The order to update.
	 * @param string   $wwpp_type 'retail'|'wholesale'.
	 */
	protected function set_order_to( $order, $wwpp_type ) {

		// $this->logger->info('Setting order ' . $order->get_id() . ' to wholesale/retail ').

		$order->add_meta_data( '_wwpp_order_type', $wwpp_type, true );

        if( 'retail' === $wwpp_type ) {
            $order->delete_meta_data('wwp_wholesale_role');
        } else {
            $order->add_meta_data('wwp_wholesale_role', 'wholesale_customer', true );
        }

		// I don't know what this is when they're not registered.
		$customer_id = $order->get_customer_id();
		if ( is_int( $customer_id ) && 0 !== $customer_id ) {
			$user  = get_user_by( 'id', $customer_id );
			$roles = $user->roles;

			if ( class_exists( WWP_Wholesale_Roles::class ) ) {
				$wholesale_roles = WWP_Wholesale_Roles::getInstance()->getAllRegisteredWholesaleRoles();
			} else {
				$wholesale_roles = array( 'wholesale_customer' );
			}

			foreach ( $wholesale_roles as $wholesale_role ) {
				if ( in_array( $wholesale_role, $roles, true ) ) {
					$order->add_meta_data( 'wwp_wholesale_role', $wholesale_role, true );
					break;
				}
			}
		}


		$order->save();

		// TODO: Add admin notice.
	}
}

Hey can you tell me where to put this code.