in my workstation hp z640, 

os version: ubuntu 20.10

steps:
1. power on workstation, press ESC >> show BIOS menu >> continute boot >> press ESC again
2. GNU GRUB menu >> select [Advanced options for Ubuntu]
3. select [Ubuntu, with Linux x.xx.x-xx-generic (recovery mode)]
4. Recovery Menu >> select [Drop ro root shell prompt]
5. remount partition / to read and write mode: mount -rw -o remount /
6. passwd root (change root's password)
7. reboot