I have done that several times like this:

Assume that in 9i your default temporary tablespace is called TEMP and the tempfile is called '/u03/oradata1/temp01.dbf' so that TEMP is the default temporary tablespace for the database.

1. CREATE TEMPORARY TABLESPACE temp2
TEMPFILE '/u03/oradata1/temp02.dbf' size 5M
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 1M;

2. ALTER DATABASE DEFAULT TEMPORARY TABLESPACE temp2;

3. DROP TABLESPACE temp;

4. CREATE TEMPORARY TABLESPACE temp
TEMPFILE '/u03/oradata1/new_name.dbf' size 1024M
AUTOEXTEND ON NEXT 256M MAXSIZE 4096M
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 1M;

5. ALTER DATABASE DEFAULT TEMPORARY TABLESPACE temp;

6. DROP TABLESPACE temp2;

Remove '/u03/oradata1/temp01.dbf' and '/u03/oradata1/temp02.dbf'
from disk.