This question already has an answer here:
Then it says:
Check your Internet connection.
W:Failed to fetch http://br.archive.ubuntu.com/ubuntu/dists/natty-backports/main/source/Sources 404 Not Found W:Failed to fetch http://br.archive.ubuntu.com/ubuntu/dists/natty-backports/restricted/source/Sources 404 Not Found W:Failed to fetch http:// br.archive.ubuntu.com/ubuntu/dists/natty-backports/universe/source/Sources 404 Not Found W:Failed to fetch http:// br.archive.ubuntu.com/ubuntu/dists/natty-backports/multiverse/source/Sources 404 Not Found E:Some index files failed to download. They have been ignored, or old ones used instead.
But I don't have any internet issues, how can I fix it?
Best Regards, Persio.