Just extract it in your Grid 2 folder, and overwrite all the files.
and before... ofc, MAKE A BACKUP of your "cars" folder!
"It seems" that it doesn't affects Tier 4 cars, have yet to play and unlock more cars to see if its all of T4 cars. D:
And, please report any other suggestion/bug u guys find, and i'll fix if i'm able to do so.