how to change the textures in some areas of map?

[justiceforall]

in all tutorials i find all the texture only add in the initial time of the game;
but I want to change texture in some specifically areas,
my map is smoothness , for example,
i assign four coordinates make a square area,
such as


(0,0,50) (50,0,50)

(0,0,0) (50,0,0)
-------->
how can i change the texture in this area when i runing the game?

can someone help me? thanks