In order to reset a Netscreen back to factory default you will need to first connect via the console connection. This is because you will lose IP connectivity once you reset the devices configuration.

You will then need to obtain the devices serial number from either of the device itself or from the CLI,

netscreen-> get system | i serial
Serial Number: 0122012008234038, Control Number: 00000000

Then using the serial number at the login prompt enter the serial number for both the user name and password.

The device will then be restored to factory default and the previous configuration wiped. To log back into the device once it has been reset use the username/password credentials of netscreen/netscreen.