I'm looking for a way to upgrade 14.10 to 15.04 - from the internet only (i.e. without using external media).
I've already tried
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Ubuntu 15.04 is not available yet and will be available in April. If you really want to test it you can download the daily build iso from here although this is highly unstable now.
After the release you can upgrade from 14.10 to 15.04 by running the following command:
I would not recommend this but if you really want to upgrade to the latest development release:
sudo do-release-upgrade -d
As Ubuntu 15.04 (Vivid Vervet) has been released on 23rd April, 2015, you can upgrade from Ubuntu 14.10 to 15.04 easily now.
At first make sure the file
/etc/update-manager/release-upgrades has the line:
Now run the following command: