sudo apt install ubuntu-gnome-desktop^
Select "gdm3" when prompted.
sudo apt remove unity lightdm ubuntu-desktop
sudo apt autoremove
sudo apt autoclean
Now, you can force to reboot the box by long pressing the shutdown button.
That's all! See you.
Update on August 19,2017 :
After the upgrade to Firefox 55.0.2, Firefox may not working properly on decimal place in your language. It shows 100,10 instead of 100.10 in my case even it is in English. This link will show you how to change it when necessary. I select "en-US" at "general.useragent.locale" in my case.