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               Lucid Lynx is the latest distro of ubuntu - one of the finest and user friendly Linux OS . This open source OS is packed with plenty of cool Features and off course promises. Currently 9.1 [ beta ]version of karmic koala was made available before 9.04 [ stable ] Karmic Koala . Upgrading your Karmic Koala 9.04 or 9.1 or any of your old Linux Ubuntu version like Jaunty Jackalope 8.04 and previous versions  to Lucid Lynx 10.04 [ LTS ] is very easy .
Method 1. If you have internet [ Fast ! ]

• Start the Update Manager (System > Administration > Update Manager)
• Now you should be able to see that a new distribution release (10.04 LTS) is available in the Update Manager. 
• Click the Upgrade button to start the distribution upgrade

Method 2. If you have no internet [ Slow internet ! ]

• You need an Offline Upgrade .
• This method is much faster as it just needs an ISO image to upgrade from HERE.
• Once you download this Image Mount the image

In terminal , type sudo mount -o loop /path-to-iso/ubuntu-10.04-alternate-i386.iso /media/cdrom0

• When success, a pop up should appear offering you the option to upgrade using that CD / ISO image.
• If for some reason the upgrade dialog doesn’t pop-up,

press ALT + F2 and type:

gksu "sh /cdrom/cdromupgrade"

And still you are unable to get ISO , then you better buy a CD , or request free CD delivered to your home free where-ever you live , and that too without any courier or shipping charges !
Just insert the fresh CD , and you will see popup that , a new version is detected , and should it proceed to update , click yes and you are done !