Note:- This is still alpha release so don't upgrade on your Desktops or laptops.If you still want to do use the following procedure
To upgrade from Ubuntu 11.10 on a desktop system: press Alt+F2 and type in "update-manager -d" (without the quotes) into the command box. Update Manager should open up and tell you: New distribution release '12.04′ is available. Click Upgrade and follow the on-screen instructions.
Download the Alpha 2
This release is for developers only. The Ubuntu Desktop images are oversized; you can use either a DVD or USB for installation instead of a CD.
You can download Alpha 2 ISOs from:
http://cdimage.ubuntu.com/releases/precise/alpha-2/ (Ubuntu Desktop and Server)
http://cloud-images.ubuntu.com/releases/precise/alpha-2/ (Ubuntu Cloud Server)
http://cdimage.ubuntu.com/ubuntu-core/releases/precise/alpha-2/ (Ubuntu Core)
http://cdimage.ubuntu.com/netboot/precise/ (Ubuntu Netboot)
http://cdimage.ubuntu.com/edubuntu/releases/precise/alpha-2/ (Edubuntu DVD)
What is new in ubuntu desktop
Until Ubuntu 11.10, the Unix group for administrators with root privileges through sudo had been admin. Starting with Ubuntu 12.04 LTS, it is now sudo, for compatibility with Debian and sudo itself. However, for backwards compatibility, admin group members are still recognized as administrators.
Automatic Apport crash reporting has been enabled by default again to catch problems early on. It now checks for duplicates on the client side, which will avoid uploading debug data and creating Launchpad bug reports unnecessarily in many cases now.
The default music player has been switched to Rhythmbox. There is no UbuntuOne Music Store yet, this still needs to be ported to GTK 3.
LibreOffice has been updated to 3.5 beta 2. Please report any regressions that you notice.
When installing new software through software-center, corresponding language support packages (translations, spell check modules particular to that software, help files, etc.) are now installed along automatically. This removes the need to open "Language Support" after installing new software.
pm-utils now has two new scripts to power down USB and various PCI devices in battery mode. A number of desktop packages were fixed to wake up less often. Both of these changes help increase battery life.
DNS resolution is now done through dnsmasq, which should help split-DNS VPNs and faster DNS resolution.
Nautilus quicklist support has been added to Unity launcher.
The control-center's "User Interface" panel now allows users to easily configure some properties of Unity.
Many compiz crashes have now been fixed.
Quicklist items can now be selected with one right click.
There are improvements with behaviour of maximized windows.
There are theming fixes in LightDM.
Minor bug fixes in Alpha 2 with new features coming in for feature freeze including proxy support and the Ubuntu One music store in Rhythmbox