follow up statement on ratings

Is there any way to leave a follow up statement, or change a rating.
I was generously refunded a partial payment when the item proved to
have many problems that the seller was not aware of.

All you can do is delete your previous comment. If the ad is still up, you can try to re-respond to the ad, which should generate another ratings ticket. As far as I know, that's the only way to re-do your comments if you feel that you didn't have complete information at the time that you rated the seller the last time. And you've probably realized this by now (as hindsight is usually 20/20), if a deal goes less than perfectly, it's probably best to wait until the deal has reached some type of conclusion before leaving a rating.