How to undo upgrade and restore previous version of Ubuntu .

You will need to reinstall if you want to use Precise. What is the issue with Quantal? Maybe we can advise.....

This is asked quite frequently, I'm not sure why you'd want to downgrade but the guide is here: https://help.ubuntu.com/community/DowngradeHowto
Make sure you backup data first as there is a high chance of all of those methods listed on that page going wrong.

I am new. I like unbuntu but right I cannot access the desktop. Down grading is too complicated for me.

What do I put after diana@diana-system-product-name:~$

The OS is called Ubuntu, not umbuntu.

What is the issue with Quantal you are having?

I know

I cannot access my desktop after the upgrade.

What video chip do you use please?

I am not sure

Which version of Ubuntu were you running previously?

The only way to downgrade is to reinstall, remembering to backup first, however you can easily get back to a simpler faster user interface "Gnome Classic"

If you want to get back to the old user interface open a terminal (CTRL+ALT+T) and enter

sudo apt-get install gnome-panel

Now next time you boot there is a small circular icon next to your user name and above the password entry box. Click on it then select "Gnome Classic" before you enter your password. It's not quite the same as the old one but its close.

Thanks