我確實可以正常訪問互聯網，並且我當前的 IP（在英國）是，82.47.29.14但我的筆記型電腦仍然連結到德國伺服器91.189.91.24 80。
但現在我無法做到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.
我應該如何解決這個問題以便我可以do-release-upgrade？