Copy from here http://askubuntu.com/questions/565576/how-to-downgrade-gnome-3-14-to-3-12-on-ubuntu-14-10
If you used this method of installing Gnome 3.14 (basically using the
ppa:gnome3-team/gnome3 PPAs – the article also details some problems that occur in Unity caused by installing 3.14), you should be able to use the following:
sudo apt-get install ppa-purge sudo ppa-purge ppa:gnome3-team/gnome3 sudo ppa-purge ppa:gnome3-team/gnome3-staging
This disables the PPAs and any reverts to the official repositories’ packages, downgrading Gnome 3.14 back to the default installed version. This should restore the bugs of large icons, Ambiance theme problems, client-side decorations etc if you want to use Unity not Gnome 3.14.
Comments: This method can apply for any “testing” purposes (install and if not okay, downgrade and remove)