Usually if you talk to whoever you have appointed minister of your court in the location you have established your court, there will be an option to change troop culture for a sum of money. You might have to appoint a companion or your wife as minister in order to gain the more advanced options like that.
This is the wrong board to post this in though lol
Edit: On second look at that particular mod it seems that while culture can be changed, for certain fiefs it bugs out or something, causing you to not receive troops of your culture, but troops of the culture the fief has originally. I don't know how to help you with that particular problem.