Always On HAG Primary Database Log Files
Can you quantify 'large'?  Are things breaking?  What's the reason you want to shrink it?

Regardless, you won't be able to switch the DB to SIMPLE mode if it's in an AG.  You'll get the following:


  Msg 1468, Level 16, State 2, Line 1 The operation cannot be performed
  on database "MyDBName" because it is involved in a database mirroring
  session or an availability group. Some operations are not allowed on a
  database that is participating in a database mirroring session or in
  an availability group. 
  
  Msg 5069, Level 16, State 1, Line 1 ALTER
  DATABASE statement failed.


What I would do is go find out if transaction log backups are being taken and their frequency.  in FULL recovery model, the log file isn't automatically truncated and cleared, you'll need to take log backups for that to happen. (In SIMPLE recovery, the log is automatically cleared so there's no need to maintain it.

If you're taking log backups, you may need to take them more frequently as the log file could be filling up between log backups. 

https://www.sqlshack.com/beginners-guide-sql-server-transaction-logs/
https://technet.microsoft.com/en-us/library/ms345583(v=sql.105).aspx