Changing A Map's Main Terrain Texture

Hi,

First of all, I would like to let you know that I am not into map creation/editing. So what I am going to ask you here may be either very easy to do or nearly impossible. I do not know.

The thing is I want to change the main terrain texture of the Nordschleife map. Either by replacing the correspondent texture or by changing its rendering size. Or whatever method is the easiest that allows me to go...

Changing A Map's Main Terrain Texture