How to change date format '17.DEC.80' to '17.12.80' in Run SQL Command Line SCOTT Database?

It depends on what 17.DEC.80 actually is; if it is a string, then to_date it first (using appropriate format model), and then apply to_char with the final format:

SQL> select to_char(to_date('17.DEC.80', 'dd.mon-yy'), 'dd.mm.yy') like_this from dual;

LIKE_THI
--------
17.12.80

SQL>

If it is a date datatype value, then just to_char it, e.g.

SQL> create table test (col date);

Table created.

SQL> insert into test values (date '1980-12-17');

1 row created.

SQL> select col from test;

COL
---------
17-DEC-80

SQL> select to_char(col, 'dd.mm.yy') like_this from test;

LIKE_THI
--------
17.12.80

SQL>

Or, alter session to set format model:

SQL> alter session set nls_date_format = 'dd.mm.yy';

Session altered.

SQL> select * from test;

COL
--------
17.12.80

SQL>

Though: we've had Y2K issue 20 years ago and learnt that 2-digits years should be avoided. I suggest you do that as well and use 4 digits for years.