Nano的镜像默认是国外的源，速度很慢，国内的源有的上不去，有的包无法安装，经过测试清华大学的源完美可用，现放上教程
首先备份原本的source.list文件
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bak #为防止误操作后无法恢复，先备份原文件sources.list
sudo gedit /etc/apt/sources.list
然后删除所有内容，复制
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic main multiverse restricted universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-security main multiverse restricted universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-updates main multiverse restricted universe
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-backports main multiverse restricted universe
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic main multiverse restricted universe
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-security main multiverse restricted universe
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-updates main multiverse restricted universe
deb-src http://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ bionic-backports main multiverse restricted universe
到sources.list后保存，之后打开终端输入
sudo apt-get update
就可以了。