Having a couple of negative feedback is normal and acceptable, as long as your account has a significant amount of feedback. If not, this will seriously hurt your standing.
In either case, you should always strive to make and keep your customers happy. You should try and resolve the issue and then after doing so you can make a request to the customer to revise the feedback. Generally, a refund (or replacement) will make your client happy.
IF the client seems amicable and is looking for a resolution, then you can send in a request to revise the feedback.
Go to My Ebay > Account > Feedback (left hand column) > Go to Feedback Forum > Request Feedback Revision (right-hand side)
Select the feedback you want to request they revise- please note that this can only be done within 30 days of the feedback being received.
You then send a note to the buyer which should include an apology, the reason for the issue (if legitimate), that you resolved it (refund...), and that you look forward to better experiences going forward and asking him to revise the feedback. The buyer can choose to or not to revise it- but please note that you can only ask one time so its important the first note is well-written.
You can request feedback revisions for a maximum of 5 feedback for every 1000 feedback in any 12 month period.