Trying to upgrade to version 11.04 from 10.10

Keep receiving this error when trying to upgrade, I have the 11.04 installer on a flash drive that I can try to upgrade with but not sure how to get it to see it as a source. Below is the error received:

W:Failed to fetch http://archive.canonical.com/ubuntu/dists/natty/Release.gpg Something wicked happened resolving 'archive.canonical.com:http' (-5 - No address associated with hostname)
, E:Some index files failed to download, they have been ignored, or old ones used instead.

When I try and run the installer (boot to it) there is no option to upgrade just install, and I would prefer not to lose my linux partition as it is the one I use 95% of the time. Any assistance would be greatly appreciated.