Ubuntu – Can’t upgrade 12.04 from 11.04

Possible Duplicate:
Failed to download repository information due to missing CDROM

I have an 11.04 machine, and I'd like to upgrade to 12.04. I'm using the Update Manager to do so. It gets to the "Setting new software channels" step when I get this error:

The complete description is:

W:Failed to fetch cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)/dists/precise/main/binary-i386/Packages  Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
, W:Failed to fetch cdrom://Ubuntu 12.04 LTS _Precise Pangolin_ - Release amd64 (20120425)/dists/precise/restricted/binary-i386/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.

What do I do?