How do I fix this?
W: The repository 'cdrom://elementary OS 0.4 _Loki_ - Stable amd64 (20160921) xenial Release' does not have a Release file. 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. E: Failed to fetch cdrom://elementary OS 0.4 _Loki_ - Stable amd64 (20160921)/dists/xenial/contrib/binary-amd64/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs E: Some index files failed to download. They have been ignored, or old ones used instead.