Instructions
Patch "Medieval: Total War 2" up to the current version. 1.4 is included with the Kingdoms expansion pack. Navigate to the directory with "Medieval: Total War 2" in it and double click on "unpack_all.bat." This extracts hidden config files, as well as other files that comprise the Medieval 2 *.dat files.
Double-click on "export_descr_units.txt" in the Medieval 2 install folder, which should default to a simple text editor. This is the file with unit descriptions in it.
Edit the variables in this file for a unit you want to modify. For the most part, the descriptors are straightforward, like "voice" and "category." Consult the guide in "References" for uses of the more obscure descriptors.