This is a common thing it seems, sellers from that country nagging buyers to change a neg until the buyer has to call eBay to end the annoyance.
If you ARE willing to change it, the seller has to send you a Request for Revision. You can't just up & change it even if you want to. The seller should know about it already:
http://pages.ebay.ca/help/feedback/revision-request.html#submit
If you are NOT willing to change it, you will probably have to use the Help & Contact link at the top or bottom of the page to call eBay and report the annoyance and get them to stop it.
FWIW, long delays in shipping are not the sellers fault. If you wanted items from China to come fast & tracked it would cost your closer to $50 -$60. Same as if you send an item there using a tracked service.
There is a thread on the Buyer board about shipping delays:
https://community.ebay.ca/t5/Buyer-Central/Asia-shipping-delays/m-p/357675#U357675