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I have ubuntu 12.04 and I want to update to 13.04, but without downloading it from the website. How can I do it ?
It's not clear if you do not want to download an iso and burn to disk, or if you just have no internet and don't want to download anything. I'm guessing the first case...and your existing system is updated this way...
For Ubuntu Desktop:
For Ubuntu Server:
13.04 Isn't an LTS so Canonical doesn't make a disk for it. Canonical typically makes install media available for their LTS (long term support) OS editions. The current LTS is 12.04.
When install media is available, it can be purchased here: http://shop.canonical.com/index.php?cPath=17