<<how do I change my feedback on a seller from last year>>
You don't. At this juncture it is not possible to change feedback. You can always add a response to that which you have already left, and any that was left for you. It will stand out on the page initially but after all this time, it is unlikely any other buyers will see it.
Worse, other sellers will see it on your outgoing feedback and fear that you are someone who is never satisfied.
Here is ebay's official blurb about warranties:
http://pages.ebay.ca/help/policies/warranties.html
There is probably nothing you can do. If you have contacted the seller to remind him that he offers a 2 year warranty and you would like compensation, but he ignores you, - then you could try to call ebay to inform them he offers warranties which he has no intention of honouring.
Like so many things here, it will doubtless come to nothing.
In future, if a product warranty is important to you, it is best to buy locally from a shop where you can return the broken item for replacement or repair whilst showing the shop clerk the warranty and product receipt.
A year after purchase there is pretty much nothing that ebay or paypal will do for a failed transaction no matter how reassuring albeit worthless the seller's promises.
