user_constraints is a view that allows you to view parts of the data dictionary that your schema owns. It's a no-no to modify the data dictionary with anything other than DDL (Data Definition Language) statements: CREATE, ALTER, DROP.
To do what you want, you'll need to drop the constraint and re-create it:
SQL> create table dlb_test (id number not null primary key, xp char(1));
Table created.
SQL> alter table dlb_test add constraint dlb_const check (xp IN ('Y','N'));
Table altered.
SQL> alter table dlb_test drop constraint dlb_const;
Table altered.
SQL> alter table dlb_test add constraint dlb_const check (xp IN ('Y','N','M'));
Table altered.
SQL>
