The answer is increase its price.
Investment transforms into higher costs of the products and given that in a monopoly the monopolist will not face competition, monopolist can take make decisions that affect the price in the short-run.
When the economy has many offerents, one firm cannot increase its price because the buyer will buy from other, so the only way to maximize profits is to produce more, decrease costs or enhancing the product (so the customers may want to pay more for a better product). In a monopoly the buyer hast not options and if the product is needed the buyer will have to pay the higher prices fixed by the monopolist.