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
首先备份原本的文件
sudo cp /etc/apt/ /etc/apt/ #为防止误操作后无法恢复，先备份原文件
sudo gedit /etc/apt/ 然后删除所有内容，复制
deb /ubuntu-ports/ bionic main multiverse restricted universe
deb /ubuntu-ports/ bionic-security main multiverse restricted universe
deb /ubuntu-ports/ bionic-updates main multiverse restricted universe
deb /ubuntu-ports/ bionic-backports main multiverse restricted universe
deb-src /ubuntu-ports/ bionic main multiverse restricted universe
deb-src /ubuntu-ports/ bionic-security main multiverse restricted universe
deb-src /ubuntu-ports/ bionic-updates main multiverse restricted universe
deb-src /ubuntu-ports/ bionic-backports main multiverse restricted universe
到后保存，之后打开终端输入
sudo apt-get update就可以了。