E: Some index files failed to download. They have been ignored, or old ones used instead.解决方案

1.删除 /var/lib/apt/lists/所有文件
$ sudo rm /var/lib/apt/lists/* -vf

有文件夹不能删除的话使用
$ sudo rm -rf /var/lib/apt/lists/*

$ sudo rm /var/lib/apt/lists/* -vf

2.再次更新
$ sudo apt-get update

3.如果还是不行，那可能就是源有问题了，我们更改源配置。
E: Some index files failed to download. They have been ignored, or old ones used instead.解决方案
在这里搜索software&&update

下面的Source code就源地址：
E: Some index files failed to download. They have been ignored, or old ones used instead.解决方案

更改下源地址，再进行1,2步骤就可以了。