you can find the translation files, which contain all the text in the whole game in the directory Medieval - Total War\Loc\Eng.
the file you are looking for (unit names) is the names.txt, while the description you see when you are building it is in the descriptions.txt file.
MAKE SURE YOU ONLY CHANGE THE TEXT BETWEEN {}s IF YOU CHANGE ANYTHING BETWEEN []s YOU WILL HOSE YOUR GAME