Check the list of participating venues on the Ticketmaster website. These venues allow a refund of your original tickets up until one week before the event if you buy new tickets of equal or greater value from Ticketmaster.
Find the tickets to which you would like to upgrade on the Ticketmaster website. Use the search form to do this quickly. Following the prompts, purchase the new tickets.
Request a refund for the unwanted tickets through your Ticketmaster.com account. If your venue is on the list of participating sites, you should be able to obtain a refund.