If you feel that the Neutral/Negative Feedback Score received is unfair, you can either:
Send a feedback revision request to your buyer to revise their feedback. However, make sure that you have communicated with the buyer and resolved their concerns first; or
If you have evidence to prove that the feedback is unfair, you can appeal to eBay to have it removed. Keep in mind that an appeal for removal of a Neutral/Negative Feedback Score needs to be done within 90 days from the date the buyer has left you the feedback.