What type of listing? If it's an auction, you'd need to end and relist it to change the starting price, but you could probably change the reserve or Buy It Now price if you had those options set up, and if you don't have bids yet. If it's a fixed price item, just click Edit Listings from your Active Listings page and you can change whatever you want including the price.
Edit: Do you have two of those trombones, or is that an accidental double listing? If you don't have both, you'd want to end one of the listings quickly to avoid them both selling and you taking a hit for an out of stock item (if you do have two, disregard this).
