vi /etc/apt/sources.list打开上面文件，控制模式下输入
:1,$d清空文件内容，然后复制粘贴以下内容：
deb http://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
## Not recommended
# deb http://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
# deb-src http://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multivers最后更新源
sudo apt-get update
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