Please be aware that a clean installation is needed if you have the previous version of the livery.
This is a modification livery derived from Formula Corsa official work.
Place the skin folder into "assettocorsa\content\cars\fc1_2009_williams_fw31\skins".
Place the skin folder into "assettocorsa\content\cars\fc1_2009_williams_fw31_s1\skins"
Hope you like it : )