SQL> create table userinfo_n 

  2  (id varchar2(10) not null,

  3  username varchar2(20));

Table created.

SQL> select constraint_name,constraint_type,status from user_constraints

  2  where table_name='USERINFO_N';

CONSTRAINT_NAME

--------------------------------------------------------------------------------

C STATUS

- --------

SYS_C00729107

C ENABLED

SQL> alter table userinfo_n

  2  rename constraint SYS_C00729107 to not_null_constraint;

Table altered.

SQL> select constraint_name,constraint_type,status from user_constraints

  2  where table_name='USERINFO_N';

CONSTRAINT_NAME

--------------------------------------------------------------------------------

C STATUS

- --------

NOT_NULL_CONSTRAINT

C ENABLED