Sql service account password change for environment on Availability groups 2012
I would do it in this order:


Change on Async Replica and restart SQL.  


Make sure everything is still synchronizing after restart

Change Sync Replica and restart.  


Make sure everything is still synchronizing after restart

Failover to Sync Replica 
Change on former Primary (Current Sync Replica), and restart.  


Make sure everything is still synchronizing after restart



You can then fail back to the old primary if need be, or leave on the Sync Replica if all things are equal.