1. 现象：（出现的错误）
执行 sudo apt-get update, 报错：apt-get 404 Not Found Package Repository Errors
执行 sudo apt-get install, 报错：Some index files failed to download They have been ignored, or old ones used
2. 解决方法：
（1）sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup
（2）sudo gedit /etc/apt/sources.list
将下面的模板内容，复制粘贴到 /etc/apt/sources.list 文件里面后，保存就可以了：（我们使用的Ubuntu 是16.04）
deb http://cn.archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse
deb http://cn.archive.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse deb http://cn.archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse deb http://cn.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse ##测试版源 deb http://cn.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse # 源码 deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial main restricted universe multiverse deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-security main restricted universe multiverse deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-updates main restricted universe multiverse deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-backports main restricted universe multiverse ##测试版源 deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse
现在，再执行：
sudo apt-get update现在就没有“apt-get 404 Not Found Package Repository Errors” 问题了。
参考文献：https://blog.csdn.net/github_35160620/article/details/52115542