Ubuntu armhf版本的源网址不同于普通Ubuntu系统，如果采用如下网址会出现问题，导致sudo apt-get update出现Error。
deb http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-security main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-updates main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-proposed main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
应该要采用如下网址：
deb http://mirrors.ustc.edu.cn/ubuntu-ports/ trusty main multiverse restricted universe
deb http://mirrors.ustc.edu.cn/ubuntu-ports/ trusty-backports main multiverse restricted universe
deb http://mirrors.ustc.edu.cn/ubuntu-ports/ trusty-proposed main multiverse restricted universe
deb http://mirrors.ustc.edu.cn/ubuntu-ports trusty-security main multiverse restricted universe
deb http://mirrors.ustc.edu.cn/ubuntu-ports/ trusty-updates main multiverse restricted universe
deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ trusty main multiverse restricted universe
deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ trusty-backports main multiverse restricted universe
deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ trusty-proposed main multiverse restricted universe
deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ trusty-security main multiverse restricted universe
deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ trusty-updates main multiverse restricted universe
具体更新方法如下，首先打开shell，输入：
cd /etc/apt
cp sources.list sources.list.backup
sudo gedit sources.list
删除文件中所有文字，将上面的网址复制到sources.list中，保存并退出。之后输入：
sudo apt-get update
等待更新结束即可。
