推荐使用清华镜像：https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/
将下列文本添加到/etc/apt/sources.list文件里
# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释 deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty main restricted universe multiverse # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty main restricted universe multiverse deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-updates main restricted universe multiverse # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-updates main restricted universe multiverse deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-backports main restricted universe multiverse # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-backports main restricted universe multiverse deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-security main restricted universe multiverse # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-security main restricted universe multiverse # 预发布软件源，不建议启用 # deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-proposed main restricted universe multiverse # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ trusty-proposed main restricted universe multiverse
或者进入repogen.simplylinux.ch 选择合适的Ubuntu版本后，在ubuntu Branches下全选
拉至网页最下，点击生产，复制list下内容，添加到/etc/apt/sources.list中即可
具体参考https://jingyan.baidu.com/article/afd8f4de66efcf34e286e993.html?st=2&os=0&bd_page_type=1&net_type=2
修改镜像源之后需要重新更新：
sudo apt-get update；sudo apt-get upgrade
如果执行sudo apt-get update仍然报错，问题在于DNS没有配置好。
解决方法：
sudo vi /etc/resolv.conf
在其中添加：
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8) # DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN nameserver 127.0.1.1 #这里用的是阿里云的DNS服务器 nameserver 223.5.5.5 nameserver 223.6.6.6