You can update existing rows using the UPDATE command. Suppose you discover the temperature readings are all off by 2 degrees after November 28. You can correct 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年11月27日
 San Francisco | 41 | 55 | 0 | 1994年11月29日
 Hayward | 35 | 52 | | 1994年11月29日
(3 rows)