I recently started playing PC again and I'm still refreshing my mind on modding etc. I replaced my police 3 files with modded files (F5544's Ford Police Interceptor 2012 Model) and I wanted to know how I can go back to the original model.

By restoring the backups you made prior to installing the modifications. If you didn't do that, you'll have to reinstall or verify your game cache (Steam). 

6 hours ago, MarkTheMarker said:

I recently started playing PC again and I'm still refreshing my mind on modding etc. I replaced my police 3 files with modded files (F5544's Ford Police Interceptor 2012 Model) and I wanted to know how I can go back to the original model.

Go find any police3 model not in the mods folder (x64e/levels/gta5/vehicles.rpf should be fine), drag and drop the original police3 files to your desktop, then go to where your modded vehicle is located and replace it with the original police3 files you put on your desktop just moment ago. And there you have the original files installed again! :)