SQL Server : How to Apply Patch in AlwaysOn Availability Group Configuration

How to Apply Patch in AlwaysOn Availability Group?

Steps for an Availability Group with one secondary replica is mentioned following and in case there are more than 1 replica too, almost same steps will be followed. 

1. Needless to mention that being a DBA, Backup is always a priority task for us. Make sure that we have taken good recent OS backup with system state (or VMware snapshot with SQL services stopped), a good recent backup of all databases and a successful completion of a checkdb on the primary node. {This is not mandatory, but to avoid “Ouch” moment}. 
2. Make note of primary replica node or just capture the screen print of cluster manager. Second, From the node acting as the primary replica (SQL1), change the failover mode to manual to ensure, auto fail-over is not being triggered. It is similar to what we do with classic cluster. In classic cluster, we remove nodes from possible owners list. 
3. Refresh the affected databases on the secondary replica (SQL2) and make sure that everything is green on the dashboard. This way, we can isolate things and troubleshooting is much easier in case we stuck with any issue. 
4. Apply the patch (service pack or CU) on SQL2  and then on SQL 3 in case we have 3 replicas. We have SQL server 2016 SP2 and CU5 to applied but steps are same for any package. 
5. Repeat the Windows Update and/or software updates until all available patches are applied ( In my case, first SP2 and then CU5). Do not move on with the patching steps until all patches and post patch reboot and configuration tasks are completed. As server reboot will be required, please ensure to apply patch on SQL 2 and then reboot that node. Once SQL 2 is up then only proceed to SQL 3.
6. Double check that patches have been applied, the cluster is healthy and AlwaysOn Availability Groups are functional. Also, check build version by running "select @@Version" and it should be the target version, In my case " Microsoft SQL Server 2016 (SP2-CU5) (KB4475776) - 13.0.5264.1 (X64)"
7. Make sure that synchronization state is SYNCHRONIZED.
8. Fail over the availability group to the secondary replica (SQL2) or to replica which was primary before starting patching activity. 
9. Refresh the affected databases on secondary Replica (former primary = SQL1) until the synchronization state is synchronized.
10. Apply the patch (service pack of CU) on SQL1.
11. Repeat the Windows Update and/or software updates until all available patches are applied. Do not move on with the patching steps until all patches and post patch reboot and configuration tasks are completed.
12. Double check that patches have been applied, the cluster is healthy and AlwaysOn Availability Groups are functional.
13. Make sure that synchronization state is SYNCHRONIZED.
14. Fail over the availability group to the primary node (back to SQL1).
15. Change the failover mode to Automatic now (which we changed in Step 2).
16. Validate all databases status and you may need to check SQL agent jobs which would have been cancelled during patching activity. 
17. Last but not least, validate SPNs of involved instances to ensure you kerborase authentication. 

Hope it will be helpful.