If you are using an Oracle Data Guard Standby database, then review the procedure you can use to downgrade the standby database with the primary database

You can downgrade a database without breaking the standby database. To downgrade Oracle Database when a physical or logical standby database is present in the Oracle Data Guard configuration, use the procedure for your downgrade scenario as described in " Patching, Upgrading, and Downgrading Databases in an Oracle Data Guard Configuration" in Oracle Data Guard Concepts and Administration