Reset User Password

ALTER USER <UserName> IDENTIFIED BY "NewPassword";

 ALTER USER scott IDENTIFIED BY "tiger";

Note double quotes used above.

-- Generate Random 16 Character Password: V1

 SELECT dbms_random.string('a', 8) || '!' || dbms_random.string('a', 7) FROM dual;

-- Generate Random 16 Character Password: V2

 SELECT translate( dbms_random.string('a', 15),
                   dbms_random.string('a',6),
                   trunc(dbms_random.value(100000,999999))) ||
        (select substr('!@#$%^&*()+<>?{}|[]',trunc(dbms_random.value(1,19)), 1) from dual)
 FROM dual;