I am guessing that you are attempting to rename your instance.
If this is true, you have to recreate your controlfile. Here are the steps to do that;

-. from server manager execute the following command :
alter database backup controlfile to trace;
-. go to your user dump destination
-. search through the list of recent file (date time stamp)
-. there should be a file there that has information about your database
-. make a copy of that file
-. if your database was called "DEV" and you wish to call it "TEST" you would
see the following:
STARTUP NOMOUNT
CREATE CONTROLFILE REUSE DATABASE "DEV1" NORESETLOGS ARCHIVELOG
-. Change it to :
STARTUP NOMOUNT
CREATE CONTROLFILE set DATABASE TEST NORESETLOGS ARCHIVELOG
-. shutdown the instance
-. Make changes to your oratab, listener, tnsnames files
-. set your oracle sid to test (export ORACLE_SID=TEST (on unix 0 or
SET ORACLE_SID=TEST (on microsoft virus))
-. start server manager and run the script.
-. When your database comes up, your instance will now be called TEST.

Hope it helps