-- first identify the existing tablespaces
SELECT tablespace_name
FROM dba_tablespaces
ORDER BY 1;
 
-- check the current undo parameters
SELECT VALUE, name
FROM gv$parameter
WHERE name LIKE '%undo%';
 
-- create the new undo tablespace 'tempundo'
CREATE UNDO TABLESPACE tempundo
DATAFILE '/u02/oradata/tempundo.dbf'
SIZE 100M REUSE AUTOEXTEND ON
RETENTION NOGUARANTEE;
 
-- now change the undo tablespace to the 'tempundo' space
ALTER SYSTEM SET undo_tablespace = TEMPUNDO SCOPE=BOTH;
 
 
-- verify the change has been made correctly
SELECT VALUE, name
FROM gv$parameter
WHERE name LIKE '%undo%';
 
 
-- finally, remove the old file/tablespace
DROP TABLESPACE <previous_tablespace_name> 
INCLUDING CONTENTS AND DATAFILES;