On RAC configuration each instance has assigned its own UNDO tablespace. So in order to resize the UNDO you must create a new one for each instance and assign it to the instance then you cane drop the old ones.
For each instance :
-- Create new undo tablespace with smaller size.
SQL> create undo tablespace UNDO_RBS1 datafile 'undorbs1.dbf' size 100m;
-- Set new tablespace as undo_tablespace
SQL> alter system set undo_tablespace=undo_rbs1;
-- Drop the old tablespace.
SQL> drop tablespace undo_rbs0 including contents.
NOTE: Dropping the old tablespace may give ORA-30013 : undo tablespace '%s' is currently in use. This error indicates you must wait for the undo tablespace to become unavailable. In other words, you must wait for existing transaction to commit or rollback.