Yeah, you do it in My Ebay. Click on your listing and it will give you some options for the reason for listing it early. It may be in the drop down menu in your selling section. Im trying to remember.
Go to My Ebay. On your Selling page, you should see your item. On the right of the listing is a drop down box. Click on the arrow. One of the options should be End Item. Click this and it will walk you through it.