Redundancy lets you perform maintenance without downtime. Here is the procedure:
Verify redundancy is working: Both systems healthy, failover tested recently.
Gracefully failover: Lower the primary priority or shut its FHRP interface. Traffic moves to backup.
Confirm traffic flow: Verify services are accessible through the backup.
Perform maintenance: Update, reboot, or replace the original primary.
Return to service: Restore configuration. Depending on preemption, traffic may or may not move back.
Schedule maintenance during low-traffic windows.