To install different vehicles:
Download whichever vehicle or pack you would like. You will be replacing police1, police2, police & police4. There are also sheriff1, sheriff2, fbi1, fbi2, pranger, ambulance & firetruck (those are all the ones I change (I think!)).
After download the files you need to get a program called OpenIV.
https://www.gta5-mods.com/tools/openiv
Open this program, click windows for GTA 5, then edit near about the top right of the page. Click into the following files: update>x64>dlcpacks>patchday13
I always use patchday13, just because it's always worked for me.
Can't remember by memory what files to click into next but after patchday13, you need to find vehicles.rpf (I'll confirm how to do this when I have access to my PC)
Click at the top of the page 'copy to mods folder'. Next drag all the police files into the vehicles folder you have open in OPENIV.
If it's an ELS comparable vehicle, you obviously need to download the ALE script from the site.
The vehicles will have .xml files to go with the corresponding police vehicle. You can drag them into the correct folder from OpenIV. Go to ELS> pack default. You can delete anything that is in the folder and replace them with the .xml files. Watch this video to get to know how to use all the ALE controls.