I am using fc6. I am not able to update or install packages using yum.
When I try to install a new package, it fails giving following error.

Loading "installonlyn" plugin
Setting up Install Process
Setting up repositories
Error: Cannot find a valid baseurl for repo: updates


How can I resolve this

Support was dropped for FC6 several years ago. There have been no official updates since that time. Update to a supported version. If you wish to stick with Fedora be aware that they only support each version for thirteen months, after that there are no updates of any kind. Current supported versions of Fedora are F9 and F10. One month after F11 is released(later this month?) support for F9 will be dropped.

Centos is RHEL(Red Hat Enterprise Linux) with the logos removed. RHEL/Centos 5.X was based on FC6 and will "feel" very familiar to you. Centos has a five year(plus) support life.