ZyXEL Communications XGS3700-24HP Release Note/Manual Supplement page 7

Upgrade XGS3700-24HP configuration:
C:\> ftp <XGS3700-24HP IP address>
User : admin
Password: 1234
230 Logged in
ftp> put 420AAGD1C0.rom rom-0
ftp> bye
Where
 User name : just press admin
 Password : the management password, 1234 by default
 420AAGD1C0.rom : the name of configuration file you want to upgrade
rom-0 : the internal configuration name in XGS3700-24HP