Upgrading Microsoft SQL 2014 and Always On AG Configurations
What you're reading is referring to changing the failover mode of an availability group replica:


  To change the failover mode of an availability replica
  
  
  In Object Explorer, connect to the server instance that hosts the
  primary replica, and expand the server tree.
  Expand the Always On High Availability node and the Availability
  Groups node.
  Click the availability group whose replica you want to change.
  Right-click the replica, and click Properties.
  In the Availability Replica Properties dialog box, use the Failover
  mode drop list to change the failover mode of this replica.
  


At this point you'll want to set the failover mode to manual. Voila!

I assume you're doing an in place upgrade by the nature of the question, so to answer your other concerns:

Removing the secondaries from the availability group would effectively do the same thing (can't fail over when there's nothing to fail over to...) but only changing the failover mode lets you keep more of the existing architecture in place without having to re-seed every replica all over again.

You also could disable the entire availability group feature in Configuration Manager, but I think that requires a restart of the service and requires you to completely setup availability groups from scratch.