Fetching the upgrade failed. There may be a network problem.

 网络连接失败：调试一下网络就好了

root@ubuntu:/usr/bin# do-release-upgrade
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [819 B]                                                                                                                                             
Err Upgrade tool                                                                                                                                                                 
  404  Not Found                                                                                                                                                                 
Fetched 819 B in 0s (0 B/s)                                                                                                                                                      
WARNING:root:file 'xenial.tar.gz' missing
Failed to fetch
Fetching the upgrade failed. There may be a network problem. 
root@ubuntu:/usr/bin# ping www.baidu.com
PING www.a.shifen.com (182.61.200.7) 56(84) bytes of data.