Changing a maps texture via lua?

Hi I was talking to the owner of Rp_EvoCity_v2d_Sexy_V2 (killerbob) he said you can change the map textures via lua.
basically I wanted to change the paths/grass to a snow texture to work with atmos, could anyone point me the right direction with there lua knowledge?

There might be a better way to do this but I generally use matproxies to achieve stuff like this.