[ubuntu] upgrade errors

Hi: i need some help. I've been trying to update - and now, upgrade - my ubuntu version.
I currently have 12.04 and am trying to get upgraded to the new 13, but am continually getting the same error message.

This is the error message:

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W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/...source/Sources 404 Not Found [IP: 91.189.91.15 80]
, W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/...source/Sources 404 Not Found [IP: 91.189.91.15 80]
, W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/...source/Sources 404 Not Found [IP: 91.189.91.15 80]
, W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/...source/Sources 404 Not Found [IP: 91.189.91.15 80]
, W:Failed to fetch http://extras.ubuntu.com/ubuntu/dist...source/Sources 404 Not Found
, E:Some index files failed to download. They have been ignored, or old ones used instead.

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any help would be appreciated.
also, I'm not a computer genius like so many other ubuntu users, so some help in laymen terms would also be awesome.
thank you very much

spencer ownside