Due to hating to use windows, I chosen Ubuntu. However, I can only login as Guest user. As you know, there are many limitations if you are using Guest account.

So I rebooted and edited boot script which used to start system in recovery mode. And it's really simple.

First, find the line started with "linux /boot/ vmlinuz" and then jump to the end of this line, delete "recovery nomodeset" and add "quiet splash re init=/bin/bash". Finally, press Ctrl + x or F10 and you are root user.

Now, you can enter "passwd root" to set a new password for root user.