Personally when I first started installing vehicles I enjoyed Kane Jr's videos on how to install vehicles.
If his videos aren't helpful to you I have made a text guide on how to replace vehicle models for you below.
If this still does not help there is plenty of people on the forums that would love to help so don't be afraid to ask.
Here is a step by step guide on how to install (replace) police car models.
This is for non-els cars! If you want me to explain that too I would be more than happy too, just let me know.
This may also seem very dumbed down. This is not to be an asshole and belittle you. I just want to make clear you know what to do.
In your GTA main directory, this is where you installed lspdfr, make a new folder by right clicking and go down to "New > folder". Name this folder "mods" (with no quotation marks)
Go back to your main directory and right click on the folder named "update" then click copy
Go back into your new "mods" folder and right click and paste or ctrl+v
After the update folder has been copied to your mods folder go online and download "OpenIV - the ultimate modding tool"
After downloading you will get something called "ovisetup". Double click on that and read what it says, and agree to the terms of service.
One of the first things OpenIV will ask for is the location of your GTAV folder. If you have GTAV on Steam the game can be found here: Steam/steamapps/common/Grand Theft Auto V.
Once you have done that click on the little box with a check mark in it in the top left area under favorites called "ASI Manager" and install the three presented items: ASI Manager, OpenIV.ASI, and openCamera.
Now that that is done click into your mods folder.
From your mods folder you want to follow this path: update/x64/dlcpacks/patchday(highest #)/dlc.rpf/x64/levels/gta5/vehicles.rpf
Now that you are here I would highly advise making a favorite of this file in the top left. Favorites act as a bookmark and make life easier next time you want to replace vehicles.
The "vehicles.rpf" file that you navigated to is where you will want to drag and drop any vehicle files you download into
If all has been executed correctly launch your game and enjoy!
If you would like me to clarify anything or explain anything in more detail please just let me know and I'll try to get back to you ask soon as possible.