Click Restart at the login window
While the computer is restarting, hold down “Command-S” until you see text scrolling through the window. This boots the computer into single user mode.
At the Localhost% prompt type:
/sbin/mount -uw /
/sbin/SystemStarter
You will then see various services starting up.
When the Localhost% prompt reappears, type:
passwd root
It will then ask you to type the new root password twice, so do so.
After entering the new password, type:
reboot