Hello,
If you have not updated the signing key & tried to upgrade the device to 6.3.0r22, you must have got the message 'bogus image' or something similar & device getting stuck in a prompt where it asks for file name, tftp IP, self IP etc.
In this case connect the first interface of the FW to TFTP server in the same lan & downgrade the ScreenOS to 6.0 release using link below:
http://kb.juniper.net/InfoCenter/index?page=content&id=KB5519&smlogin=true&actp=search
Once done, before upgrade delete the key using 'delete crypto auth-key' or install new key and then proceed to do upgrade. For ISG & NS5K devices, bootloader upgrade is also mandatory.
Regards,
Rushi