1.At the boot loader menu, use the arrow keys to highlight the installation you want to edit and type [A] to enter into append mode.
2.You are presented with a prompt that looks similar to the following:
grub append> ro root=LABEL=/
3.Press the Spacebar once to add a blank space, then add the word single to tell GRUB to boot into single-user Linux mode. The result should look like the following:
ro root=LABEL=/ single
4.Press [Enter] and GRUB will boot single-user Linux mode. After it finishes loading, you will be presented with a shell prompt similar to the following:
5.You can now change the root password by typing
You will be asked to re-type the password for verification. Once you are finished, the password will be changed. You can then reboot by typing reboot at the prompt; then you can log in to root as you normally would.