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I'm running 10.04 and want to upgrade.
I've downloaded 12.04 to my machine (not a disc) and can't get it installed.
How do I get it installed?
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You can easily upgrade over the network with the following procedure.
For 10.04LTS users you need to check the "Release upgrade - Show new distribution releases" drop-down to make sure "Long term support releases only" is selected, and change it if otherwise. See the Trouble-shooting section below for more details. Please see the trouble-shooting section for the special case for LTS users between 12.04 and the release 12.04.1
https://help.ubuntu.com/community/PreciseUpgrades for more information on some alternate methods of upgrading for certain cases.
Do you want a fresh install or an upgrade? In any case you need to make a startup USB disk from the iso file you've downloaded using usb-creator-gtk. I suggest you back up your home folder and make a fresh install using a startup disk. (You will have to install your applications afresh but backing up the home folder enables you to save the settings.)