It is simple.....Just for the start , the most simple and basic would be like this-
1.Make sure u have all the texture to be switched in the memory .That means , load all your textures when initially creating the world .
2.Apply your first texture on the object while creating world/scene .
3. For switching texture -->
Have your touch/button clicking condition checked (Like in helloworld sample see the touchturn("if "condition) or something like that in the ondrawframe ).
Inside that ,set the second texture on the desired object in the ondrawframe . But yes ..you have to make sure that you do that only once or it would throw an error .
You can do that by having a boolean global variable set to false ( secondtexapplied=false)..And then inside your button/touch logic in ondrawframe ....
if(secondtexapplied==false)
{
change/apply second tex on object
secondtexapplied==true;
}