If the total was added today you should be able to use the rain today editor.
Click on Edit then Today's rain to open.
If it happened on another day, then edit the total in dayfile.txt. You can use
the built in editor for this as well. Click on Edit then Dayfile.txt.
Also, if you have an incorrect "yesterday" rainfall amount you will need to fix
this manually in today.ini located in the data directory. Make sure you stop Cumulus if you need to do this.
Hope this helps.