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I am trying to upgrade through the "Update Manager." After clicking upgrade the "Distribution Upgrade" dialog appears and begins, "Preparing to Upgrade" at which point I get a message:

Third party sources disabled
Some third party entries in your sources.list were disabled. You can re-enable them after the upgrade with the 'software-properties' tool or your package manager.

I click close and the "Distribution Upgrade" moves on to "Setting New Software Channels." At that point I get:

Error during update

A problem occurred during the update. This is usually some sort of
  network problem, please check your network connection and retry.

W:Failed to fetch 404 Not Found [IP: 80]

W:Failed to fetch 404 Not Found [IP: 80]
W:Failed to fetch h t t p:// 404 Not Found [IP: 80]
W:Failed to fetch h t t p:// 404 Not Found [IP: 80]

E:Some index files failed to download. They have been ignored, or old ones used instead.

The upgrade then fails.

Any help appreciated.

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possible duplicate of How can I fix a 404 Error using a PPA? – devav2 Nov 15 '12 at 20:07

It's more complicated to upgrade from 10.04 to 12.04 via 10.10, 11.04, 11.10, etc., because many of those releases are long since EOL and unsupported. I haven't figured out how to do it myself.

However, you can upgrade from LTS to LTS, from 10.04 directly to 12.04, skipping in the intermediary EOL releases. You have to set Update Manager to only look for Long-Term Support Releases though.

Follow the instructions here:

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