1) Reboot the box and press SPACE when you see the following screen to get to the loader prompt.

writing to flash...
Protected 1 sectors
Loading /boot/defaults/loader.conf
/kernel data=0x9f5c18+0xd8d68 syms=[0x4+0x7e350+0x4+0xb49c7]

Hit [Enter] to boot immediately, or space bar for command prompt.
Booting [/kernel] in 1 second...

Type '?' for a list of commands, 'help' for more detailed help.
loader>

2) if you hit space when you see

Hit any key to stop autoboot: 

you will get the prompt

Octeon srx_300_ram# 

Type the command

boot <ENTER>

and wait for the

Hit [Enter] to boot immediately, or space bar for command prompt.

3) Type “boot -s” in this prompt

loader>boot -s

4) After a while you will see a screen like below. Type “recovery” to continue

System watchdog timer disabled
Enter full pathname of shell or 'recovery' for root password recovery or RETURN

NOTE: If you don’t see this prompt for recovery but instead “Enter root password”, then password recovery must have been disabled via the configuration “set system ports console insecure” which means you can’t recover the password via this method.

5) In the end you will see that you will drop to the shell without any password. Once you type configure and followed by the “set system root-authentication plain-text-password”, you will have the new password assigned. Don’t forget to commit

NOTE: Once in the CLI, you will need to enter configuration mode using
NOTE: the 'configure' command to make any required changes. For example,
NOTE: to reset the root password, type:
NOTE:    configure
NOTE:    set system root-authentication plain-text-password
NOTE:    (enter the new password when asked)
NOTE:    commit
NOTE:    exit
NOTE:    exit
NOTE: When you exit the CLI, you will be asked if you want to reboot
NOTE: the system

Starting CLI ...
root> configure 
Entering configuration mode

root# set system root-authentication plain-text-password 
New password:
Retype new password: