Power-cycle the device and wait at the console for the message:
"Hit [Enter] to boot immediately, or space bar for command prompt."
Press space bar immediately!
Then at the prompt "loader>" type: "boot -s" to execute single user boot.
loader> boot -s
When again presented with the prompt:
"Enter full pathname of shell or 'recovery' for root password recovery or RETURN for /bin/sh:" type "recovery"
Then enter configuration mode and issue instruction for root password change and commit the changes:
configure
set system root-authentication plain-text-password
(enter the new password when asked)
commit
issue couple of times exit command till system asks "Reboot the system? [y/n]"
Then agree to reboot by pressing y and and press [Enter] key.
Done.