Download the latest version of the software and copy it over to the SRX
scp junos-srxsme-12.3X48-D65.1-domestic.tgz root@192.168.1.1:/var/tmp
On the SRX install the software into the alternative root partition
request system software add /var/tmp/junos-srxsme-12.3X48-D65.1-domestic.tgz no-copy no-validate unlink
Reboot
request system reboot
Once it has rebooted, update the alternate image to the new version.
request system snapshot slice alternate