I can't upgrade from Ubuntu11.10 to 12.04

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http://askubuntu.com – when I try to upgrade the system ,it always failed. W:无法下载 http://cn.archive.ubuntu.com/ubuntu/dists/precise-updates/main/binary-i3... 404 Not Found , W:无法下载 gzip:/var/lib/apt/lists/partial/cn.archive.ubuntu.com_ubuntu_dists_precise_universe_i18n_Translation-en Encountered a section with no Package: header , E:Some index files failed to download. They have been ignored, or old ones used instead. what should i do ? (HowTos)