1. Boot OS until boot menu appears.
2. Stop boot process. (Press any arrow keys)
3. Press a to append parameters to boot command line.
4. After space bar type rhgb quiet 1 and press ENTER.
5. Press b to boot.
6. After loading of operating system in single mode (without any password!) you can change root password by typing passwd
7. Reboot the server and test that the new root password is working.