How do I retrospectively change a grading for a review I've written?

Hi all. I've posted a review for an item I cancelled. No negatives, just didn't buy it so there was no need for a review request to appear! But it did and I completed it. I labelled the the review 'neutral' because I wasn't purchasing anything. But the seller was helpful so I think I should have labelled it 'good'. How can I retrospectively change a review grading?