change user

If you got the message like this:

$ su - root
Password: 
su: Authentication failure
Sorry.

Then you can input the command:
sudo passwd root 
open the root user and input the passwd:

$ sudo passwd root
Enter new UNIX password: 
Retype new UNIX password: 
passwd: password updated successfully

so that you can su -root

Want to close the root user in the ubuntu, run the command:
sudo passwd root -l