When I try to update elementary OS it gives me this error:
W: GPG error: http://us.archive.ubuntu.com/ubuntu precise-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5
E: The repository 'http://us.archive.ubuntu.com/ubuntu precise-backports InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://us.archive.ubuntu.com/ubuntu precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5
E: The repository 'http://us.archive.ubuntu.com/ubuntu precise Release' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.