使用apt-get安装时，会很慢，更换了国内的源后，就可以解决这个问题了。
1. 备份sources.list文件
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
2. 打开sources.list文件
sudo gedit /etc/apt/sources.list
3. 删除原内容，添加下列内容
#清华源 deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-updates main restricted universe multiverse deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-updates main restricted universe multiverse deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-security main restricted universe multiverse deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-security main restricted universe multiverse deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-backports main restricted universe multiverse deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-backports main restricted universe multiverse deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic main universe restricted deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic main universe restricted
#中科大源 deb http://mirrors.ustc.edu.cn/ubuntu-ports/ bionic-updates main restricted universe multiverse deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ bionic-updates main restricted universe multiverse deb http://mirrors.ustc.edu.cn/ubuntu-ports/ bionic-security main restricted universe multiverse deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ bionic-security main restricted universe multiverse deb http://mirrors.ustc.edu.cn/ubuntu-ports/ bionic-backports main restricted universe multiverse deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ bionic-backports main restricted universe multiverse deb http://mirrors.ustc.edu.cn/ubuntu-ports/ bionic main universe restricted deb-src http://mirrors.ustc.edu.cn/ubuntu-ports/ bionic main universe restricted
这两个源都可以用，我都试过，感觉中科大源速度更快。
4. 更新
保存sources.list文件后需要更新apt-get。
sudo apt-get update
更新完成后就可以正常使用了。