My oracle primary server is crashed, then how to switch all users to standby oracle database ? also how to rebuild primary database using standby database ?

SQL>RECOVER STANDBY DATABASE;
SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE FINISH;
Activate standby database:
SQL> ALTER DATABASE ACTIVATE PHYSICAL STANDBY DATABASE;
Check New Status from V$DATABASE VIEW .
SQL> SELECT OPEN_MODE,PROTECTION_MODE,DATABASE_ROLE FROM V$DATABASE;
Open Database
SQL> ALTER DATABASE OPEN;
SQL> SELECT OPEN_MODE,PROTECTION_MODE,DATABASE_ROLE FROM V$DATABASE;