Navigate to Grand Theft Auto\Grand Theft Auto V\update\x64\dlcpacks\787b\dlc.rpf\x64\vehicles.rpf\

open the "787b.YTD' file 

Make sure your game is fully closed 

Drag and drop the contents from a livery folder into the texture editor. Be sure to hit save in the bottom right corner

Upon starting the game you should see the new liviery on the car 
