I'm not sure what you mean by "changing models" but if you mean changing the game's already created models, then click here to get the models.
Inserting a custom model to replace the original model is what I mean. I already have the models.
Do you mean modding the game so the enemies look different? I can't help you if that's the case. :p
Not modding, physically changing the files. For example, you copy your custom model, then paste it into the folder with the original file, like the MK 4 swordbot. Then, you delete the MK 4 swordbot model, and you have your own custom model for the MK 4 swordbot in your own game.