xmodulo.com/how-to-fix-apt-get-update-error-on-ubuntu.html
如果是依赖没找到，可以用 sudo apt-get install -f 先补齐依赖看行不行。
1、先使用：
$ sudo rm -rf /var/lib/apt/lists/* $ sudo apt-get update
如果不行，再使用：
2、
$ sudo rm -R /var/lib/apt/lists/partial/* $ sudo apt-get update
如果还不行，则
3、
Then check out the "End of Life" date of your Ubuntu release by referring to https://wiki.ubuntu.com/Releases
如果你的ubuntu版本已经 end of life了，则表示不再被支持，即软件库、更新库就不可用了
但是你可以使用 old-releases：
ask.xmodulo.com/404-not-found-error-apt-get-update-ubuntu.html
Every Ubuntu release has its end-of-life (EOL) time; regular Ubuntu releases are supported for 18 months, while LTS (Long Term Support) versions are supported up to 3 years (server edition) and 5 years (desktop edition). Once a Ubuntu release has reached EOL, its repositories will no longer be accessible, and you won't get any maintenance updates and security patches from Canonical. As of this writing, Ubuntu 13.04 (Raring Ringtail) has already reached EOL.
If the Ubuntu system you are using is EOL-ed, you will get the following 404 errors from apt-get or aptitude as its repositories have been deprecated.
W: Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/raring-backports/multiverse/binary-i386/Packages 404 Not Found [IP: 91.189.91.13 80] W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/raring/main/binary-amd64/Packages 404 Not Found W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/raring-security/universe/binary-i386/Packages 404 Not Found [IP: 91.189.88.149 80] E: Some index files failed to download. They have been ignored, or old ones used instead
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/utopic-security/main/source/Sources 404 Not Found W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/utopic-security/restricted/source/Sources 404 Not Found W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/utopic-security/universe/source/Sources 404 Not Found W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/utopic-security/multiverse/source/Sources 404 Not Found W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/utopic-security/main/binary-amd64/Packages 404 Not Found
For those users who are using old versions of Ubuntu, Canonical maintains old-releases.ubuntu.com, which is an archive of EOL-ed repositories. Thus, when Canonical's support for your Ubuntu installation ends, you need to switch to repositories at old-releases.ubuntu.com (unless you want to upgrade it before EOL).
Here is a quick way to fix "404 Not Found" errors on old Ubuntu by switching to old-releases repositories.
First, replace main/security repositories with old-releases versions as follows.
$ sudo sed -i -r 's/([a-z]{2}.)?archive.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list $ sudo sed -i -r 's/security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
Then open /etc/apt/sources.list with a text editor, and look for extras.ubuntu.com. This repository is also no longer supported for 13.04. So you need to comment out extras.ubuntu.com by prepending '#' sign.
#deb http://extras.ubuntu.com/ubuntu raring main #deb-src http://extras.ubuntu.com/ubuntu raring main
Now you should be able to install or update packages on an old unsupported Ubuntu release.
4、大多数时候采用 sudo add-apt-repository *** 添加的repo source，很多时候在 apt-get update 的时候都会失败，可以这样删掉刚添加的repo：
sudo add-apt-repository ppa:whatever/ppa sudo apt-get update ....fail..... sudo add-apt-repository --remove ppa:whatever/ppa