View your rated music, apps and etc. on the iTunes?
Follow these simple steps to remove a review that you have written:


Sign in to the iTunes Store.
From the Store menu, choose “View My Account” (or click “Account” in the “Quick Links” menu on the right).
In the Settings section, click “Manage” next to “Reviews and Ratings”.



You’ll end up on this page:


Click “Remove” next to the review or rating you would like to remove.


Note: The review that you remove will not immediately disappear. Once removed, the text from the review will no longer be visible but may be retained in Apple’s systems indefinitely.

Source: Apple’s Knowledge Base