Step 1:

Navigate to your Root Folder Location and open the "content" folder

Step 2:

Rename the "cars" folder to "old_cars"

Step 3:

Open steam and Verify game files for Assetto Corsa

Step 4:

Once the verifying files is complete return to your Root Folder Location and again open the "content" folder and check to make sure verifying files created a new "cars" folder

Step 5:

Load the server you want to play on and use How to use "Install Missing Content" to download cars