It’s quite easy to do it, just install the Gnome fallback session package and then you get the Gnome Classic in Login menu at next time.
To install the package, press Ctrl+Alt+T on your keyboard to open terminal. When it opens, run below commands in terminal:
sudo apt-get install gnome-session-fallback
Or you can search for and install gnome-session-fallback in Ubuntu Software Center.
After that, log out and select the Classic session to log back.