I freely admit that I am not a metal work person and that I am no expert at all, but many people wait years/decades for their copper to "turn" to brown and verdigris. This is generally a wanted change. People apply accelerators to their shiny copper or brown weathered copper to make this slow oxidation faster than the decades it really takes naturally. You could use a copper cleaner/polisher and/or some steel wool or sandpaper to restore the shiny copper look, but you'll have to apply some kind of urethane or finish to keep it from turning brown and starting the patina process over again. Plan on using plenty of elbow grease.