附上清华大学源的发布地址（该站包含Ubuntu、CnetOS、Fedora等主流Linux发行版下各版本的清华源）：
第1步获取清华源信息：
打开下面的连接选择对应的系统版本，将会在页面生成对应的源信息
https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/
https://mirrors4.tuna.tsinghua.edu.cn/help/ubuntu/
第2步，备份原来的源文件：
#备份原来的源文件
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
第3步，写入清华源:
cat>/etc/apt/sources.list <<-EOF
# 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/ xenial main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse# 预发布软件源，不建议启用
# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-proposed main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-proposed main restricted universe multiverse
EOF
第4步更新源：
apt-get update