Hi George,
The stats should update as you edit the data - if they don't, quit WeatherCat then navigate (in the Finder) to your WeatherCatData folder (this is at ~/Library/WeatherCatData where ~ is your home directory - more info in the manual) - navigate to the year folder of the data you were editing and delete the monthly stats file for that month - this file ends in a '.hrs' extension (it's the hourly data). For example if you were editing data in June 2006, you would delete the 6_WeatherCatData.cat.hrs file in the 2006 folder. When you reboot WeatherCat it'll rebuild the stats from the raw data.
Cheers,
Stu.