How to use DBNEWID utility to change DBID of database ? (only from 9i onwards)

Ensure that the target database is mounted but not open, and that it was shutdown consistently prior to mounting.

1) Start database in mount mode

SHUTDOWN IMMEDIATE
STARTUP MOUNT

2) To change database name and DBID, this example changes the name to testdb_N1:

nid TARGET=SYS/oracle@testdb DBNAME=testdb_N1

or To change database name only :-

nid TARGET=SYS/oracle@testdb DBNAME=testdb_N1 SETNAME=YES
Change database id of database TEST? (Y/[N]) => y

3) After DBNEWID successfully changes the DBID, shut down the database:

SHUTDOWN IMMEDIATE

4) Mount the database and Open the database in RESETLOGS mode :-

STARTUP MOUNT
ALTER DATABASE OPEN RESETLOGS;