今天打开虚拟机进行 sudo apt-get update时候，提示 NOT FOUND IP:XXX.xx.xx.但实际是可以打开的，然后又尝试进行安装软件，果然，还是Not found ip，最后想了一下，决定对镜像源进行更新。
这里用的是中科大源，更改的文件在 /etc/apt/sources.list中，首先先备份源文件 sudo cp /etc/apt/sources.list /etc/apt/sources.list .bac k
然后编辑 sudo vim /etc/apt/sources.list，把里面的全都注释掉，注释方法为在前面加 # ，然后讲一下内容粘贴上去即可。
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
deb https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src https://mirrors.ustc.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
最后，执行
sudo apt-get update
sudo apt-get upgrade