Db2 11.1

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CREATE tABLE test (te CHAR(125))
INSERT INTO test VALUES('12345678901234567890234567890')
SELECT SUBSTR(TE,14,1) FROM test

1

4

SELECT
  REPLACE(TE, SUBSTR(TE,14,1), 'N')
FROM test;

1

123N567890123N56789023N567890

SELECT REGEXP_REPLACE(
  TE,SUBSTR(TE,14,1), 'N', 13,1,'c' )
      FROM test

1

1234567890123N567890234567890