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

The character is probably made up of textured surfaces.

You will need to assign a texture to a blitz brush and then paint the individual surfaces of the mesh.

i.e. something along the lines of:
surf = getsurface(GentChar,1) ; Could be a number of surfaces!
Br = GetSurfaceBrush(Surf)
BrushTexture Br,Texchar
PaintSurface Surf,br