To avoid having issues with loosing sudo access to display respect the following step to modify the computer name in Ubuntu 16.04.

First, edit the hostname by typing the following command in the terminal: sudo gedit /etc/hosts. It will prompt you to enter the password. (The name to be change should be on the second line)

Then, edit the hostname by typing the following command in the terminal : sudo gedit /etc/hostname

Finally change the computer name clicking in the top right corner and on About This Computer.

The error:

Failed to connect to Mir: Failed to connect to server socket: No such file or directory
Unable to init server: Could not connect: Connection refused
Error: cannot open display: :0

can be solved by the command xhost +. Be aware that there might be an issue with the hostname that led to this issue (check the previous step to change the computer name).

Creating a new user can be achieved through the GUI. Press the super key and look for user accounts.

Click unlock and add an administrator account. Put a password by clicking on account disabled. Ubuntu might want an elaborate password. It can later be changed using the command sudo passwd adeye.