If you are logged in, just go and find your review, and click "Edit this review" to make changes. Please keep in mind that your review will need to be re-approved, so it will temporarily disappear from the site.

This is a very useful feature if, as you use the product more, you begin to change your mind about its effectiveness, or you have more information to contribute. While the system allows you to put in multiple reviews for a product, you might just want to make a small edit or not have to put another review in just to counter an earlier rating.