How to alter a subpartition template

Use the SET SUBPARTITION TEMPLATE clause, like this:

ALTER TABLE SCOTT.MYTABLE
SET SUBPARTITION TEMPLATE
  (SUBPARTITION SP_2014 VALUES ('2014') TABLESPACE USERS,
   SUBPARTITION SP_2015 VALUES ('2015') TABLESPACE USERS,
   SUBPARTITION SP_2016 VALUES ('2016') TABLESPACE USERS,
   SUBPARTITION SP_2017 VALUES ('2017') TABLESPACE USERS,
   SUBPARTITION SP_2018 VALUES ('2018') TABLESPACE USERS,
   SUBPARTITION SP_2019 VALUES ('2019') TABLESPACE USERS,
   SUBPARTITION SP_2020 VALUES ('2020') TABLESPACE USERS,
   SUBPARTITION SP_DEFAULT VALUES (default) TABLESPACE USERS
  );

Source: Oracle Documentation