Summary: This article documents the recommended steps for updating search appliances in a mirroring configuration.
Details: In mirroring only the primary search appliance crawls, but primary and replica(s) search appliances can serve. As such you should always be able to have at least one search appliance serving. The offical update documentation best describes the update process, however it is important to note that mirroring should be disabled prior to attempting the process. First update the replicas and then finally update the primary. For each search appliance follow the process below:
Log in to the search appliance, navigate to Google Search Appliance > GSA^n and disable mirroring.
Perform the update procedure for all the GSAs, as described in the official documentation.
Log back into to the search appliance, navigate to Google Search Appliance > GSA^n and reconfigure mirroring.