In the Great Britain mod folder, look for the LeaderheadInfos.xml file, I think, in the Text/Civilizations folder. Or maybe it was CivilizationInfos.xml...it's been awhile since I've looked at it.
In any case, just poke around in the Text folder until you find the right file, and then use Ctrl-F to find the leader you want (some of those files are 1000's of lines long). Then, scroll down to the traits, and type in what you want. Finding it is the hard part--the editing is quite easy.