Well you can phone Help and Contact and they may allow you to change your feedback but it is unlikely.
I don't see any negs on your FB page , so I guess this is for another account.
Is your seller promising a replacement or a refund if you change the feedback? Tell him you will try to make the change AFTER you have the refund. Don't bother with a replacement, it won't arrive. Ever.
Did you leave FB prematurely? Allow a good 20 days for items arrive (30 for Asia but no more). Then contact the seller. Then if that is unsatisfactory go to the Resolution Centre.
You have 60 days to leave feedback. Take your time.
If you can't change the FB and the problem has been resolved, you can leave a Response to your own and to your seller's FB indefinitely. The most effective Responses, like the most effective FB is calm and factual.