Here is the upgrade procedure for HA Pair:
1. Disable Sync at Slave (unit #2) unit
2. Remove Slave (unit #2) unit from network
3. Upgrade Slave (unit #2) to new firmware
4. Re-connect Slave (unit #2) unit to network
5. Remove Master (unit #1) from network
6. Upgrade Master (unit #1) to new firmware
7. Re-connect Master (unit #1) back to network
8. Enable Sync at Slave (unit #2)