Is it possible to install Gnome 3 (relased on 6th April)on Ubuntu 10.04 or 10.10 ?
And if i can can can i revert back to the earlier version ?
There may be experimental repositories for Gnome 3 for 10.04 or 10.10, but I can assure you Ubuntu won't offer it as a direct upgrade and most likely will not support it. Depending on how the packages are built, you might be able to get back to Gnome 2, but don't expect it to work flawlessly. If you're really itching to try it, wait for the next release of Ubuntu (11.04) where they have it available via PPA, but again, it's highly experimental. Have a look at this link:
In System -> Administration -> Software Sources add the repository ppa:gnome3-team/gnome3. Open Synaptic from the Administration menu as well and click Refresh in the upper left. Go into the Origin sort in the lower left and select the GNOME3 ppa (the name will be notably different from any already there). Install gnome-desktop3 and any dependencies and restart. This will probably break GNOME2 so perhaps install XFCE4 (or any other desktop environment) at the same time so you can revert if GNOME3 doesn't work. Then note on the GNOME3 page on Launchpad says,
"This package contains packages from GNOME3 and their dependencies so they can be used in Ubuntu 11.04 (Natty). This PPA is EXPERIMENTAL and MAY BREAK YOUR SYSTEM. There is no downgrade process."
so be careful.
Changing desktop environments is along the bottom of the screen after you select your username.
To revert, you would go into Software Sources again and remove the source we added earlier, although it will now be called "deb http://ppa.launchpad.net/gnome3-team/gnome3/ubuntu natty main" or something similar. Open Synaptic and refresh packages again. Go into the Status tab in the lower left and mark any packages under Local / Obsolete that seem GNOME-related for reinstallation.
Do all this with caution.
If it is available, it should be in your repositories. Just search for it in Synaptic. When you log in, you get the choice of different desktop environments. (Gnome 2.X, 3, Kde, Xfce)
This is afaik, I haven't been on Ubuntu since 10.04 was new.