Firmaware Upgrade in HA mode

Take a look at the online help. There you will find more information about the update process.
https://docs.sophos.com/nsg/sophos-firewall/18.0/Help/en-us/webhelp/onlinehelp/HighAvailablityStartupGuide/HAFirmwareUpgrade/index.html 

In HA, a group of two devices works as a single entity known as a cluster. Every HA cluster has one primary device and one secondary (auxiliary) device. The primary device controls how the cluster operates, while the auxiliary device always waits to become the primary device. The roles of the primary and auxiliary devices in the cluster depend on the configuration mode.

When you upgrade an HA device, the process is as follows:

1. The primary device (device A) upgrades the secondary device (device B).
2. Device B runs the new firmware and takes control of the network. It's now the primary device and device A is the secondary.
3. Device A then upgrades and runs the new firmware. It's still the secondary device, but if you have configured the other device as a preferred primary, then the cluster will failover.

Hello Vaibhav Patil ,

Thank you for reaching out to the community, Please refer the Firmware upgrade and pattern updates.