First Find a Car you like
Download it from the LSPDFR page or a website of your choice.
Extract it (Personally I Use WINRAR)
Youube videos on how to use that
Once you have extracted your folder open, openIV (download from internet)
Once you have your vehicle folder and open iv open go into edit mode. Button on the top.
Use the read me in the vehicle folder to tell you where to go!
E.G Update/X64/DLCpacks (in the read me its should tell you what dlc pack to go into)
patchday3ng/ dlc.rpf/x64/levels/gta5/vehiclesrpf/ find the car your replacing (should tell you in the read me file. and select those files (Example police.2) find all the files (police 2 ) and highlight them and then press the green plus button at the top of the screen. After selecting the green cross find where you extracted vehicle files and select all of them.
Simple (kind off) now your new cop car should be ready in game for you.
Hoped this helped if not message me and i will walk you through it..