How do you make your ranks full again after a battle in medieval 2 total war

Not 100% sure this is what your talking about, but it sounds like you are talking about retraining. You do this by going to the city where your army is and open the production window and clicking on the retraining tab. Just select the units you want to retrain
NOTE: You must have the building to produce that unit in the city. The way I normally do it is buy grouping units together as much as possible to cut down on the cost

Hope this helps