1. On the OS selection screen, press 'E'.
2. type in :
rw init=/bin/bashthen press 'ctrl + x'.
3. You will brought to a console, wait it to fully load and, type in :
passwd -u rootpress 'enter'. Then type in :
passwd rootpress 'enter'.
4. Next, you will be prompt for the new password. Take note that, you would not see the cursor moving while you type. Type the new password and press 'enter'. Then enter the password again for confirmation, then press 'enter'.
You are done. Restart your computer and try to login as root.