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!