Thomas Rushton has a checklist for upgrading servers connected by an Availability Group:
This is a checklist put together and followed for an upgrade of a pair of physical SQL Server 2012 servers which hosted a single availability group of several terabytes of data with minimal downtime.
The availability group was configured with synchronous commit and automatic failover.
Minimizing downtime here is great, but it’s not automatic: you still need to do work on your end to get this right.