There are three basic ways to retexture and reskin an object in Unity: using the Unity editor, using scripting, and using external tools.
The Unity editor is the simplest way to retexture and reskin an object. Simply select the object, then in the Inspector window select the desired texture or material.
Scripting is the most powerful way to retexture and reskin an object. With scripting you can change the texture or material of an object at runtime, or even dynamically generate textures and materials.
External tools can also be used to retexture and reskin objects. For example, you can use Photoshop or other image editing software to create textures, and 3D modeling software to create materials.