You want to put your NSMXpress Juniper Appliances into a factory default state.
This howto will show you how:
Connect a console device to the console port of the NSMXPress appliance.
Physically power off the NSMXpress appliance.
Power the NSMXpress appliance back on.
During the boot sequence the console will display an option to "press any key to enter the menu". Hit any key. When the boot sequence is interrupted, a menu will display on the console. Choose Re-Install
then at the password prompt choose: "erase".
That's it, the Juniper NSMXpress Management console will format the harddrive.