Which member of a high-availability cluster should be upgraded first in a Zero downtime upgrade?

 In a Zero downtime upgrade, you should upgrade the Standby Member first. This is because the Standby Member does not process traffic and can be upgraded without affecting the cluster availability. After upgrading the Standby Member, you can perform a failover and make it the Active Member. Then you can upgrade the original Active Member, which becomes the Standby Member after the failover. References: Getting Started - Check Point Software, section “Upgrading Cluster Members with Zero Downtime”