我正在尝试从 Ubuntu 14.04 升级到 Ubuntu 16.04，但在升级过程中，系统显示
Error during update
A problem occurred during the update. This is usually some sort of network problem, please check your network connection and retry.
W:Failed to fetch http://archive.ubuntu.com/ubuntu/dists/stable/main/binary-amd64/Packages 404 Not Found
, W:Failed to fetch http://archive.ubuntu.com/ubuntu/dists/stable/main/binary-i386/Packages 404 Not Found
, E:Some index files failed to download. They have been ignored, or old ones used instead.