为防止误操作后无法恢复，先备份原文件
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak
新建新的源文件source.list
sudo gedit /etc/apt/sources.list
删除文本框所有内容
复制清华源
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic main restricted universe multiversedeb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-updates main restricted universe multiversedeb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-backports main restricted universe multiversedeb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-security main restricted universe multiverse
更新一下源：
sudo apt-get update