CREATE UNDO TABLESPACE undotbs2
DATAFILE '/01/oradata/dbname/undotbs201.dbf'
SIZE 50M AUTOEXTEND ON NEXT 5M;

Tablespace created.

ALTER SYSTEM SET UNDO_TABLESPACE=UNDOTBS2 SCOPE=BOTH;

DROP TABLESPACE undotbs1 INCLUDING CONTENTS AND DATAFILES;

*
ERROR at line 1:
ORA-30013: undo tablespace 'UNDOTBS1' is currently in use

SYS@ecs01> alter tablespace undotbs1 offline;

Tablespace altered.
SYS@ecs01> DROP TABLESPACE undotbs1 INCLUDING CONTENTS AND DATAFILES;

Tablespace dropped.