Following are the steps to flashback primary database in Dataguard

1. Cancel media recovery on the standby database.

ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;

2.  Shutdown primary database.

shutdown immediate;

3.  Start primary database in mount stage.

STARTUP MOUNT

4. Flashback primary database to a specified scn

FLASHBACK DATABASE TO SCN <scn_number>;

5. Open primary database with reset logs.

ALTER DATABASE OPEN RESETLOGS;

6. Check current_scn on the standby database, flashback scn is greater than current_scn then skip the next step (step 7).

SELECT CURRENT_SCN FROM V$DATABASE;

7.  Flashback Standby database

FLASHBACK STANDBY DATABASE TO SCN <scn_number -2>;

8. Start managed recovery on the standby database.

ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT FROM SESSION;