Ocorrem erros quando executo o apt-get update.
GPG error: http://old-releases.ubuntu.com/ubuntu raring Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32 W: The repository 'http://old-releases.ubuntu.com/ubuntu raring Release' is not signed. N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use. N: See apt-secure(8) manpage for repository creation and user configuration details.
você adicionou manualmente alguns repositórios externos (não oficiais) que impedirão que você atualize a lista de pacotes
edite /etc/apt/sources.list ou qualquer outro arquivo na pasta /etc/apt/sources.list.d/ e remova qualquer referência a link deles