is it possible to remove or change feedback once left?

i recieved an item today and it looked fine so i left good feedback. on closer inspection i have discovered it was not as described.

it was a some test strips for aquarium water, in the advert it showed a bottle of 100, i recieved a bottle of 25. they look identical.

ive contacted the seller via ebay messages and am waiting on a reply.

not planning to do anything as yet regarding the feedback but was just wondering if i can change it if the seller ignores me.