HelloAR: The Andy.OBJ cannot be replaced

I tried to modify the "Andy.OBJ" in HelloAR e.g. using the 3D Builder of Win10, but not successed.
May I know what software I should uses? e.g. it come together with a .MTL which I don't know how to uses in the MainActivity.cs to call.

And, any reference for creating geometry in OnDrawFrame? I want to connect all Anchor to form a polygon on surface but don't know how to do it.

Thanks