If you're using UltimateBackup, go into the UltimateBackup folder (Grand Theft Auto V>plugins>LSPDFR>UltimateBackup) and open the files named DefaultRegions.xml and SpecialUnits.xml. Then, simply search up "policeb" using Notepad (or whichever text editor you might use) and replace that with whichever vehicle you want to use.
If you're using the LSPDFR menu, go into the data folder (Grand Theft Auto V>lspdfr>data) and open the file named agency.xml. Again, search "policeb" using your text editor of choice and replace it.
This process may depend on the mods you're using, but if you're running default LSPDFR and/or default UltimateBackup, this is how to change them.