If you want to change a Neg to a neut or positive you need to ask the seller to send a request to revise feedback.

If you want to change a Positive to a neg or neut, you can not do this,

You can add to the feedback with follow up feedback left but it will not change the +ive rating.

If you are changing because the seller has asked you to, DON'T change it until you have received refund or the seller has fixed the problem.

Whatever the issue may be, ONLY change it after issues have been solved. 😉