Ubuntu 18.04 will use GNOME to replace Unity as its default Desktop environment. Though Unity 7 will remain supported until 2021, you can now install Gnome Shell alongside in Ubuntu 17.04.
Mark Shuttleworth, Founder of Ubuntu and Canonical, wrote on this blog post:
As we head into the new fiscal year, it’s appropriate to reassess each of our initiatives. I’m writing to let you know that we will end our investment in Unity8, the phone and convergence shell. We will shift our default Ubuntu desktop back to GNOME for Ubuntu 18.04 LTS.
To install Gnome 3 Desktop (Gnome Shell), open terminal via Ctrl+Alt+T, when it opens, run command:
sudo apt install gnome-shell
Type in password (no visual feedback) when prompts and hit Enter.
After that, log out and select login with Gnome session: