Change or switch undo tablespace in Oracle database

$ sqlplus / as sysdba

SQL> show parameter undo

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_management                      string      AUTO
undo_retention                       integer     900
undo_tablespace                      string      UNDOTBS1
SQL> create undo tablespace undotbs2 datafile '/data/data_files_xe/undo02.dbf' SIZE 4G;
alter system set undo_tablespace= undotbs2 SCOPE=BOTH;

drop tablespace UNDOTBS1 including contents AND DATAFILES;