Foto 3D is designed to generate textured geometry in one quick operation. Using it for geometry generation only, and then replacing the texture in Design, is a bit awkward, but it can be done.
You can simply delete the texture in Design 3D. However, because of the way Foto 3D creates the geometry and the image map, you will need to redefine the UV mapping before you will be able to apply new textures (and not get an unexpected result.)
The way to do this is to bring the model from Foto into Design as a vrml file, then Export it as an Obj file. This should strip out the UV mapping.
Then you can bring the model back into Design 3D to redefine the UV mapping using the UV tools in Design 3D CX 7. Any texture maps you then apply will have correct UV mapping, and will behave as you expect.