Download software
https://www.juniper.net/support/downloads/?p=srx320#sw
Upgrade procedure:
https://kb.juniper.net/InfoCenter/index?page=content&id=KB16652&actp=METADATA
Steps:
sftp file to /cf/var/tmp/
https://www.juniper.net/support/downloads/?p=srx320#sw
https://kb.juniper.net/InfoCenter/index?page=content&id=KB16652&actp=METADATA
sftp file to /cf/var/tmp/