Once you have installed Ubuntu 14.04 in your system and would like to change the theme to Gnome Classic Desktop this post will help you,

Step 1:
Open your terminal and logged to super user,

Step 2:
Execute the below command,

#    sudo apt-get update;

#    sudo apt-get install gnome-session-fallback

Once completed the Installation logged out from current user session, click on Ubuntu logo.

You will see few themes,
GNOME Classic
GNOME Flashback (Compiz)
GNOME Flashback (Metacity)
Ubuntu (Default)

Choose “GNOME Classic”,

Now, you are working with GNOME Classic Desktop.