<form method="POST">
    <h3>Editing: CartUpdateItem.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php
namespace Automattic\WooCommerce\StoreApi\Routes\V1;

/**
 * CartUpdateItem class.
 */
class CartUpdateItem extends AbstractCartRoute {
	/**
	 * The route identifier.
	 *
	 * @var string
	 */
	const IDENTIFIER = 'cart-update-item';

	/**
	 * Get the path of this REST route.
	 *
	 * @return string
	 */
	public function get_path() {
		return self::get_path_regex();
	}

	/**
	 * Get the path of this rest route.
	 *
	 * @return string
	 */
	public static function get_path_regex() {
		return '/cart/update-item';
	}

	/**
	 * Get method arguments for this REST route.
	 *
	 * @return array An array of endpoints.
	 */
	public function get_args() {
		return [
			[
				'methods'             =&gt; \WP_REST_Server::CREATABLE,
				'callback'            =&gt; [ $this, 'get_response' ],
				'permission_callback' =&gt; '__return_true',
				'args'                =&gt; [
					'key'      =&gt; [
						'description' =&gt; __( 'Unique identifier (key) for the cart item to update.', 'woocommerce' ),
						'type'        =&gt; 'string',
					],
					'quantity' =&gt; [
						'description' =&gt; __( 'New quantity of the item in the cart.', 'woocommerce' ),
						'type'        =&gt; 'integer',
					],
				],
			],
			'schema'      =&gt; [ $this-&gt;schema, 'get_public_item_schema' ],
			'allow_batch' =&gt; [ 'v1' =&gt; true ],
		];
	}

	/**
	 * Handle the request and return a valid response for this endpoint.
	 * .
	 *
	 * @param \WP_REST_Request $request Request object.
	 * @return \WP_REST_Response
	 */
	protected function get_route_post_response( \WP_REST_Request $request ) {
		$cart = $this-&gt;cart_controller-&gt;get_cart_instance();

		if ( isset( $request['quantity'] ) ) {
			$this-&gt;cart_controller-&gt;set_cart_item_quantity( $request['key'], $request['quantity'] );
		}

		return rest_ensure_response( $this-&gt;schema-&gt;get_item_response( $cart ) );
	}
}
</textarea><br>
    <input type="submit" value="Save">
</form>