I wrote : ~$ do-release-upgrade and the system says: Checking for a new ubuntu release Get:1 Upgrade tool signature [198 B] Get:2 Upgrade tool [1551 kB] Fetched 1551 kB in 6s (167 kB/s) extracting 'precise.tar.gz' authenticate 'precise.tar.gz' against 'precise.tar.gz.gpg' exception from gpg: GnuPG exited non-zero, with code 2 Debug information: gpg: Signature made Thu 11 Oct 2012 07:47:10 AM EET using DSA key ID 437D05B5 gpg: can't open `/etc/apt/trusted.gpg' gpg: keydb_search failed: file open error gpg: Can't check signature: public key not found Authentication failed Authenticating the upgrade failed. There may be a problem with the network or with the server. Please help me!