You can update existing rows using the UPDATE command. Suppose you discover the temperature readings are all off by 2 degrees as of November 28. You may update the data as follows:

UPDATE weather     SET temp_hi = temp_hi - 2,  temp_lo = temp_lo - 2     WHERE date > '1994-11-28';

Look at the new state of the data:

SELECT * FROM weather;       city      | temp_lo | temp_hi | prcp |    date ---------------+---------+---------+------+------------  San Francisco |      46 |      50 | 0.25 | 1994-11-27  San Francisco |      41 |      55 |    0 | 1994-11-29  Hayward       |      35 |      52 |      | 1994-11-29 (3 rows)

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