In other post I explain Howto remove unity and install gnome3. but since gnome3 is unstable this will change unity into default gnome but a stable version :
1) Click on the power button in the upper right corner (mine looks like a light switch) and choose the last option, System Settings.
2) Search for Login Screen
3) Double-click to display
4) Choose Unlock and enter your password
5) Select Ubuntu Classic as default session.
Please comment if you have problems.