安装Ubuntu 18.04后，默认源在国外，可以替换为国内的源以提升访问速度

参考https://mirrors.ustc.edu.cn/repogen/

sudo vi /etc/apt/sources.list，写入以下内容

deb https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse

deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse

deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse

deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse

## Not recommended
# deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
# deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse

保存，然后执行sudo apt-get update && sudo apt-get upgrade