How do I change a texture for the cube object using the unity script.

Hi, Thank you for reading this post. I'm a beginner for the unity and the unity script. I have some questions for the unity script.

[Question No1] When I want to change a texture on the surface of the cube object dynamically, how do I change a texture for the cube object using the unity script. At initial time, some textures (i.e. the assets) are already imported my project.

[Question No2] The textures(image files) will be produced by the outside of the unity project. How do I import a texture to my project dynamically using the unity script.

I use the JavaScript but the C# is also accepted. Please let me know any suggestions or answers. Thanks.