Unless something has changed, as a buyer, you cannot change your feedback unless the seller specifically requests that you do so. He or she must submit a request through ebay to you to do so.
What you can do, however, is 'leave a response' as follow-up to feedback left. You need to be logged into your account on a desktop computer or in a web browser. Basically you go to your own feedback page and there should be link at the bottom. In this respect, you won't be able to change the rating ( + or - ) but you will be allowed to add another line to what you've already left. Changing a negative to a positive can only be done at the request of the seller.
If you're in a position where you left a positive and now regret it, you might want instead to visit the Resolution Centre to address any Return or Not as Described issues you might be facing.
