Upgrade Ubuntu from Edgy to Feisty

Well, I upgrade my ubuntu to Feisty by using "update-manager -c -d".
But an error occurred.
If you use GUI to upgrade it, you might get the following message:

Authentication failed
Authenticating the upgrade failed. There may be a problem with the network or with the server.

or use command line to type "sudo update-manager -c -d", get more specific message:

extracting ‘/tmp/tmpVuEU0n/feisty.tar.gz’
authenticate ‘/tmp/tmpVuEU0n/feisty.tar.gz’ against‘/tmp/tmpVuEU0n/feisty.tar.gz.gpg’
exception from gpg: GnuPG exited non-zero, with code 131072

It's obvious that we don't have a gpg authentication. So, just type gpg ,as a normal user. Once it says Go ahead and type your message …, you can just press Ctrl-C to get back to the command prompt.