Once you are logged into the DiamondBistro, if you click on the "my active listings" button, then from there you should see a menu bar featuring the options of Title, Sold? and Statistics.
Click on the 'Manage This Listing' button beneath the item you wish to edit. From there, there is a button that looks like a white square of paper and a pencil, click on that to edit the individual listing.
Do not create a duplicate listing, or end your listing and create a new one with the new price as duplicated listings are typically flagged and removed.