From Fedora Project Wiki
Boot from the install or live CD/DVD
Select "Rescue installed system"
Answer the prompts for language and keyboard.
Starting the network is optional and not needed.
Let the rescue mode mount your file systems in the read write mode.
Hit enter to get the root shell prompt.
# chroot /mnt/sysimage (Change to your disk file system.) # vi /etc/grub.conf (Editing boot loader configuration file)
Locate the password line and remove it (press dd on vi in command mode to delete a line)
Save the file then do:
# sync ( changes are written to disk)
# exit
Congratulations! You now have now reset your boot loader password. The system will now unmount the file systems and reboot.