I havent gone in and done much editing in the last few years, its an aggravation I just dont need.  I used to open the .rtf file that stores the letters and invoices in WordPad and edit it, it was a more user friendly environment.

The file name you want will be in the ProSeries2024 >Common Folder the standard invoice is named  fdiinvc.rtf Im not sure what file name it would be if it was a custom one.