http://www.hamweather.com/kb/index.php?article=21

The easiest way to do this is using phpMyAdmin (another webapp), which lets you modify the MySQL database. To do this, you need the 4 letter ICAO abbreviation for the airport closest to you. If you live in the US, it's going to be "K" followed by the 3-letter airport abbreviation - i.e., for Gainesville, it's "KGNV" instead of just GNV.

An alternative is to register HAMWeather - http://www.hamweather.com/hw3/store/, in which case they'll send you a complete database that is already updated.