14.04 - How to upgrade ubuntu if i get 'Authenticating the upgrade failed'?

I want to upgrade my ubuntu 14.04 distribution to the 16.04.

I'm working in a domain buy I have sudo permissions, the repositories point to our private repositories

I try to upgrade with the command do-release upgrade and the application and I got this error.

$ sudo do-release-upgrade
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [198 B]
Get:2 Upgrade tool [1262 kB]
Fetched 1262 kB in 0s (0 B/s)
authenticate 'xenial.tar.gz' against 'xenial.tar.gz.gpg'
gpg exited 1
Debug information:
gpg: Signature made dc 26 oct 2016 16:32:27 CEST using DSA key ID 437D05B5
gpg: /tmp/ubuntu-release-upgrader-gzuou970/trustdb.gpg: trustdb created
gpg: BAD signature from "Ubuntu Archive Automatic Signing Key "
Authentication failed    Authenticating the upgrade failed. There may be a problem with the network or with the server.

And when I try to upgrade through Software updater I get this error.

How could I do this?