You need to find the vehicle model that was used when making this texture. Once you have acquired this, open openiv. If you are replacing the Police.ytd model then go to x64e.rpf/levels/gta5/vehicles.rpf. Make sure the edit mode is on. Drag then new police model you are wanting to change then located the police.ytd file. Open it and find where the skin is, most often then not it will be something along the lines of explorer_sign1. You will just have to go through the files and find it. Once found click replace then replace the skin, ensure to save it. Once that is done I like to open the .yft file to make sure the skin fits well and there aren't any errors. After that run the game and it should be good to go.
New Bitmap Image.bmp