You can directly contact the Rider to request changes if the Rider has not placed your order at the restaurant. Some other conditions can be applied, for example, new items must have the same price or higher than the old ones, and you may have to pay more.
You cannot change the items of the orders placed at the BAEMIN Partner, however, you can call Customer Service to make a request of canceling and make another order, as long as the restaurant has not prepared the dishes for your order.