You can do that from SSMS GUI, drill down the the Avaialbility group then right-click properties. On the General Page there is a section for Availability Replica where all the replica is listed. On the column Readable Secondary you can change it to the setting you want. Or by TSQL:
ALTER AVAILABILITY GROUP [AGName] MODIFY REPLICA ON N'InstanceName'WITH(SECONDARY_ROLE (ALLOW_CONNECTIONS = READ_ONLY));
You can change read_only to the setting you would like.