Can I upgrade Ubuntu 16.04 LTS to Ubuntu 18.04 LTS from already downloaded 18.04 LTS ISO ?
If can, How?
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The official upgrade from LTS to LTS is possible when the 1st point release gets released (18.04.1). Current target for the release of the 1st point release is July 26th.
You can do
sudo do-release-upgrade -d on a fully updated 16.04 if you want to upgrade from command line. Only do that when you know how to restore your backup.
Due to the changes between 16.04 desktop (unity) and 18.04 desktop (gnome 3), I would re-install anyways. Make sure you can restore a backup for your personal data.