How i can change or switch a texture of a 3DS object plz.

MD2 and B3D format support deformable meshes - 3DS and X don't. So the only way you can animate 3DS and X, is to either A) Convert it to B3D, or B) keep it segmented (for example, the upper and lower arm would have to be a separate object).

I'd recommend converting to B3D.