我确实可以正常访问互联网，并且我当前的 IP（在英国），82.47.29.14但我的笔记本电脑仍然连接到德国服务器91.189.91.24 80。
但现在我无法做到do-release-upgrade：
alvas@ubi:~$ sudo do-release-upgrade -d
Checking for a new Ubuntu release
Err Upgrade tool signature
404 Not Found [IP: 91.189.91.24 80]
Err Upgrade tool
404 Not Found [IP: 91.189.91.24 80]
Fetched 0 B in 0s (0 B/s)
WARNING:root:file 'utopic.tar.gz.gpg' missing
Failed to fetch
Fetching the upgrade failed. There may be a network problem.
alvas@ubi:~$ sudo do-release-upgrade -d
Checking for a new Ubuntu release
Err Upgrade tool signature
404 Not Found [IP: 91.189.91.23 80]
Err Upgrade tool
404 Not Found [IP: 91.189.91.23 80]
Fetched 0 B in 0s (0 B/s)
WARNING:root:file 'utopic.tar.gz.gpg' missing
Failed to fetch
Fetching the upgrade failed. There may be a network problem.
我该如何解决这个问题才能do-release-upgrade？