You can relist items on your Selling Dashboard. Click on the Actions menu of your listing, and you'll be presented with two separate options for relisting your item.
These options are available whether it has expired or is still active.
Listing Fees still apply on all relists, as you are essentially creating a new listing. There is no credit or proration for relisting an active listing before it expires.
Relist takes your active listing and makes it new. It retains its previous Listing ID, your listing will be eligible to appear on New Today and the homepage since your start and end date are reset. Some users prefer this option to "refresh" their listing after a week or two of being active, as it gets their listing back into areas of the site with the most traffic.
Create New Listing
Create New Listing is for when you have multiple or similar items of an active listing that you want to sell, and do not want to use the quantity feature. This option creates a separate listing with a new listing ID. No changes are made on the original listing.
You can also have your listing automatically relisted if it does not sell. Though you can add this option during listing creation, you may also add it any existing listing after the fact by selecting the Edit Listing option from the Actions drop-down. Read more about Auto Relist.