Hi
Using any of the Fedora Live Media
Boot the Live installation media
After it finishes booting and starts the live session, open a terminal and switch to root (using su, it won't ask for a password)
Create a directory where you can mount the filesystem of your installation:
mkdir /mnt/sysimage
Mount the filesystem of your installation (/dev/sda1 is just an example, be sure to fill in the actual device node of your installation root / partition):
mount /dev/sda1 /mnt/sysimage
chroot to your installation:
chroot /mnt/sysimage/
Change the root password:
passwd
Exit from the chroot:
exit
That's it, simply reboot your system and then boot the installation from the HDD as usual.