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)