Firstly, contact the buyer directly and try to resolve the issue. Whilst buyers can’t edit an Amazon feedback, they can delete it and add new reviews.
Negative feedback may be justified and may highlight an area of improvement you can make so future customers do not experience the same issue. Take a positive out of a negative and learn from your mistake.
As with getting top feedback, ensure communication is both timely and polite. It is nice when to get a personal response showing empathy when a problem arises rather that an automated response.