If you've scratched your Xiaomi Yi camera lens, this video will show you how to replace it. It's not too hard, and the replacement lens is only around $15.
Purchase the replacement lens here:
To replace the lens, first remove the front plate of the camera with a plastic separator tool. Then remove the hot glue that holds the lens in place with a needle-nose pliers or hemostat or something like that. Screw out the damaged lens and screw in the replacement. Now focus the lens. Once the lens is focused, put a little bit of hot glue in place to hold it in place. Finally, reassemble the lens.
It might be a good idea to do the Yi microphone mod while you have the camera open. Here is a video that shows how to do that.