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I know you can re-review an item, say if they make a bunch of changes and want you to review the latest version. I know I've even done this myself. My memory is that you go to the item you reviewed, scroll down to the bottom. Where it lets you clear the rating you gave, that is your ticket to be able to re-do the review. Seems to me you can't change the original review, but you can review it again. Just clear the rating, and you should be able to do that.
Give that a shot, and in the meantime, maybe someone way smarter or experienced will drop by and either tell you I'm full of horse feathers or I actually told you the correct way to do it.