Where or which tool can we export more map tiles to "Assets/Textures/NaturalEarthII"

I’m starting to code with Cesium and I would like to have more tiles map for deeper levels to a specific area. My idea is to add more map tiles to “Assets/Textures/NaturalEarthII”.
What do I need to do indeed?

I also have a source of map tiles in TMS. I’m not sure if Cesium can use it or not?