Unpack the archive in your left for dead 2 folder, usually C:\Program Files\Steam\steamapps\common\left 4 dead 2 demo\left4dead2
MAKE A BACKUP of C:\Program Files\Steam\steamapps\common\left 4 dead 2 demo\left4dead2\pak01_dir.vpk
Open pak01_dir.vpk with a proper text editor (i recommend Notepad++ or similar)
Replace
$baseTexture "models\survivors\mechanic\mechanic_body_color"
to
$baseTexture "models\survivors\mechanic\bechanic_body_color"
Replace
$baseTexture "models\survivors\mechanic\mechanic_head_color"
to
$baseTexture "models\survivors\mechanic\bechanic_head_color"
Thats it, now intead of loading the original textures packed in the game's files, the game will load the ones you copied in the materials folder.