使用sudo apt-get install update时，出现如下错误：
`E: Some index files failed to download. They have been ignored, or old ones used instead.`
具体错误内容为：
...
...
Ign http://hk.archive.ubuntu.com trusty/restricted Translation-en_HK
Ign http://hk.archive.ubuntu.com trusty/universe Translation-en_HK
Fetched 2,580 kB in 9s (269 kB/s)
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-updates/main/source/Sources Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-updates/universe/source/Sources Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-updates/main/binary-amd64/Packages Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-updates/universe/binary-amd64/Packages Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-updates/main/binary-i386/Packages Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-updates/universe/binary-i386/Packages Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-backports/universe/source/Sources Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-backports/universe/binary-amd64/Packages Hash Sum mismatch
W: Failed to fetch http://hk.archive.ubuntu.com/ubuntu/dists/trusty-backports/universe/binary-i386/Packages Hash Sum mismatch
E: Some index files failed to download. They have been ignored, or old ones used instead.
原因及解决办法：
所使用的源镜像地址被墙掉了，换个国内的镜像地址。
方法：
修改/etc/apt/sources.list,将其中的hk.archive.ubuntu.com替换成cn.archive.ubuntu.com（电信服务器，ubuntu中国官方镜像，maintained by Shanghai Linux User Group）
sudo vim /etc/apt/sources.list
:%s/hk.archive/cn.archive/g
也可以换为其他镜像源，比如：
mirrors.163.com
mirrors.sohu.com
ftp.sjtu.edu.cn #上海交通大学服务器
debian.ustc.edu.cn #中科大
mirror9.net9.org #清华
mirror.rootguide.org #上海电信
...
（各个镜像在速度上有所差别，校园网用户推荐使用教育网镜像。）
试过的其他解决办法，对于我的情况来说都无效，但有部分网友回复可以顺利解决他们的问题：
方法1：
sudo rm /var/lib/apt/lists/* -vf
sudo apt-get update
方法2：
gpg –keyserver keyserver.ubuntu.com –recv 3E5C1192
gpg –export –armor 3E5C1192 | sudo apt-key add -
sudo apt-get update
方法3：
sudo sh -c "echo 'Dir::Ignore-Files-Silently:: \"（.save|.distupgrade)$\";' > /etc/apt/apt.conf.d/99ignoresave"
方法4：
Go into Software Sources > Other Software and remove the conflicting ppa's.
(我用的服务器，没图形界面，这个方法就不知道有效没了，见评论里说是有效的。)
方法5：
sudo apt-get -f install
...