修改apt-get源为国内镜像源

# 备份
cp /etc/apt/sources.list /etc/apt/sources.list.bak
sudo vim /etc/apt/sources.list

# 阿里镜像
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse 
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse 
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse

# 中科大
deb http://mirrors.ustc.edu.cn/ubuntu/ precise-updates main restricted
deb-src http://mirrors.ustc.edu.cn/ubuntu/ precise-updates main restricted
deb http://mirrors.ustc.edu.cn/ubuntu/ precise universe
deb-src http://mirrors.ustc.edu.cn/ubuntu/ precise universe
deb http://mirrors.ustc.edu.cn/ubuntu/ precise-updates universe
deb-src http://mirrors.ustc.edu.cn/ubuntu/ precise-updates universe
deb http://mirrors.ustc.edu.cn/ubuntu/ precise multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ precise multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ precise-updates multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ precise-updates multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ precise-backports main restricted universe multiverse

# 搜狐源
deb http://mirrors.sohu.com/ubuntu/ precise-updates main restricted
deb-src http://mirrors.sohu.com/ubuntu/ precise-updates main restricted
deb http://mirrors.sohu.com/ubuntu/ precise universe
deb-src http://mirrors.sohu.com/ubuntu/ precise universe
deb http://mirrors.sohu.com/ubuntu/ precise-updates universe
deb-src http://mirrors.sohu.com/ubuntu/ precise-updates universe
deb http://mirrors.sohu.com/ubuntu/ precise multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-updates multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-updates multiverse
deb http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://mirrors.sohu.com/ubuntu/ precise-backports main restricted universe multiverse

# 网易源
deb http://mirrors.163.com/ubuntu/ precise-updates main restricted
deb-src http://mirrors.163.com/ubuntu/ precise-updates main restricted
deb http://mirrors.163.com/ubuntu/ precise universe
deb-src http://mirrors.163.com/ubuntu/ precise universe
deb http://mirrors.163.com/ubuntu/ precise-updates universe
deb-src http://mirrors.163.com/ubuntu/ precise-updates universe
deb http://mirrors.163.com/ubuntu/ precise multiverse
deb-src http://mirrors.163.com/ubuntu/ precise multiverse
deb http://mirrors.163.com/ubuntu/ precise-updates multiverse
deb-src http://mirrors.163.com/ubuntu/ precise-updates multiverse
deb http://mirrors.163.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ precise-backports main restricted universe multiverse

apt-get update
apt update
sudo apt-get update
sudo apt-get upgrade