I have the same problem as you, specifically to fix all the rain totals relatively easily. One place to fix them is to right click Rainfall on the main page and go to both locations that show up. The only way I’ve found to permanently correct yesterday’s rain is to edit tag prevdayrain in wdisplay.ini.
You can look at this topic (FAQ: How do I correct bad data?) on this page http://discourse.weather-watch.com/t/13427. I have tried to follow this FAQ but have met with only limited success.
Your success in this endeavor hopefully will also result in my success as I am presently quite disgusted.