1. 現象：（出現的錯誤）
執行 sudo apt-get update, 報錯：apt-get 404 Not Found Package Repository Errors
執行 sudo apt-get install, 報錯：Some index files failed to download They have been ignored, or old ones used
2. 解決方法：
（1）sudo cp /etc/apt/sources.list /etc/apt/sources.list_backup
（2）sudo gedit /etc/apt/sources.list
將下面的模板內容，復制粘貼到 /etc/apt/sources.list 文件里面后，保存就可以了：（我們使用的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 ##測試版源 deb http://cn.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse # 源碼 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 ##測試版源 deb-src http://cn.archive.ubuntu.com/ubuntu/ xenial-proposed main restricted universe multiverse
現在，再執行：
sudo apt-get update現在就沒有“apt-get 404 Not Found Package Repository Errors” 問題了。
參考文獻：https://blog.csdn.net/github_35160620/article/details/52115542