Make sure to have OpenIV and then make sure to use the "Edit Mode".

After that, Open the mods folder, go to mods>update>x64>dlcpacks>11john11_lspd_pack>dlc.rpf>x64>vehicles_main.rpf and put the liveries to pscout.ytd 
(if you want, you may put the Leonard County livery to the LSSD Pack made by john, in that case go to mods>update>x64>dlcpacks>11john11_lssd_pack>dlc.rpf>x64>vehicles.rpf and open "sheriffscoutnew.ytd"
For the modern Leonard County livery, you'll need the 2020 Vapid Scout Cruiser Utility by Nacho. go to mods>update>x64>dlcpacks>dnscout>dlc.rpf>x64>vehicle.rpf and open the "dnscout.ytd" file, after that, you can either replace the already pre-installed livery, or you can just make it as a add-on livery. Same thing goes for the white VDPD livery.

MAKE SURE TO HAVE THE LSPD PACK MADE BY 11JOHN11 INSTALLED, OTHERWISE YOU CANNOT INSTALL THE LIVERIES. SAME THING GOES FOR THE LSSD PACK AND FOR THE 2020 SCOUT!

LSPD Pack by 11john11: https://www.gta5-mods.com/vehicles/lspd-lssd-pack
LSSD Pack 2.0 By 11john11: https://www.gta5-mods.com/vehicles/los-santos-county-sheriff-department-pack
2020 Vapid Police Cruiser Utility: https://www.gta5-mods.com/vehicles/2020-vapid-police-cruiser-utility