使用apt-get安裝時，會很慢，更換了國內的源后，就可以解決這個問題了。
1. 備份sources.list文件
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
2. 打開sources.list文件
sudo gedit /etc/apt/sources.list
3. 刪除原內容，添加下列內容
#清華源 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
這兩個源都可以用，我都試過，感覺中科大源速度更快。
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。