This article provides the pre-requisite steps to be taken before the upgrade on SRX series chassis cluster.

The below steps can be followed to decide on upgrade path, image selection from Juniper downloads page and upgrade process.

1. The upgrade path can be selected as per this rule, from any release, you can jump a maximum of three (3) releases up/down and from EEOL releases you can jump a maximum of two (2) EEOL releases up/down. Please refer Junos upgrade paths for SRX platforms for more details. To know whether the version is EOL/EEOL, refer Junos OS Dates & Milestones.
2. Once the upgrade path is selected as per the above rule, login into Juniper downloads site and enter the device model you want to upgrade. Select the OS as Junos if the upgrade is to be performed to specific release or select Junos SR, if upgrade is supposed to be done to specific service release.
3. There're three methods to upgrade SRX series chassis cluster and those are as follows, simultaneous reboot, minimal downtime and ISSU. Please refer [SRX] How to upgrade Junos OS on a Chassis Cluster for more details.