I can't update ubuntu 12.04 LTS to Ubuntu 12.04 LTS _Precise Pangolin

view story

http://askubuntu.com – Hy! I can't update my ubuntu, Update Manager shows this: CD/DVD 'Ubuntu 12.04 LTS Precise Pangolin - Release i386 (20120423)' is required Please insert the above CD/DVD into the drive '/media/cdrom/' to install software packages from it. The image I downloaded and burned to cd is from http://releases.ubuntu.com/12.04/ the PC (Intel x86) desktop CD version I burned many cd's but the result is the same. (HowTos)