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
Method 1: download from the SRX ( if internet access is available )
get the source link from the juniper download website
file copy "https://cdn.juniper.net/software/junos/18.3R1.9/junos-srxsme-18.3R1.9.tgz?SM_USER=jkriker&__gda__=1538133362_08dde5d15a6a7599f99df990b77adea1" /var/tmp
request system software add no-copy /var/tmp/junos-srxsme-11.4R4.4-domestic.tgzrequest system reboot
Method 2: download from the laptop browser then copy to the SRX
sftp file to /cf/var/tmp/
request system software add no-copy /var/tmp/junos-srxsme-11.4R4.4-domestic.tgzrequest system reboot