SQL server always ON patching Always on(SQL server 2016) is configured between four standalone servers Server1,Server2,Server3,Server4. These servers contains one availability Group(AG01) which contains 3 databases. How to apply SQL server patch for these servers? Do we need to apply patch for all servers at same day? These servers are production servers.
The process to upgrade/patch Always On Availability Groups is documented extensively in the official SQL Server docs. Typically this is done as a rolling upgrade, where you move from secondaries to the primary SQL Server. Depending on your setup you would do the following:


Patch remote secondaries
Patch local secondaries
Perform a failover of the primary
Patch former primary


You don't typically do them all the same time, you roll through the servers. When I patch our servers, I normally do the secondaries one day and then the following, I'll perform the failover and finish patching the former primary.