[Closed] ebay, changing feedback you've already given

I've ordered something from fleabay. It's a new-old-stock item. It's electrical.

It arrived promptly. I left nice feedback.

I've since tested it, and found it not to be working.

Having traced the fault I think I can fix it with a 10p component.

However.

I'm fairly sure I would like to change my feedback to "not as described". On account of it not working.

I've never changed feedback before - is it possible? Is it worth it? Could I aim for a partial refund? After all, I have had to spend an hour or so of my own time debugging this thing, which was never described as "spares/repair only" - if it was I wouldn't have bought it.

Help me, STW hive mind.