How do I extract the terrain file from a texture pack to remodify it

[Elixium]

So, I do got the default terrain file:

How can I extract a file like that from a texture pack to remodify its style.


Thanks in advance

 

[Peb]

Hello, Used to be a resource pack creator Pebbers#1553 if you need any help
Dm on here or Dm on discord